Note: Here's a reply from a 24in iMac owner (I assume):

" The 24in is a totally different animal. The front bezel is a breeze to pull off. Just get rid of the screws on the bottom and lift it. Disconnect the iSight and micro connections and there you are.
In addition being it larger components have more space and the monitor can be lifted after pulling the 8 torx screws and the lateral grounding(?) tape.
Changing HD or DVD drive is easy but for the rest there is little to be done since I suppose one needs to pull the main frame (main logic board) out to get to the processor or other innards.
-Saluti "

(Earlier notes follow)

" Some people have expressed interest in the disassembly of the new iMacs. This is not a complete guide but it should get you started if you can not find the Apple service manual online although I would dissuade ANYONE from attempting this due to the difficulty and potential for damage involved. Keep in mind any damage done to your computer during your adventures inside void the warranty and could result in expensive repairs. (Disassembly is at your OWN RISK.)
With the case design change that incorporates the iSight into the chassis the iMac went from being trivially simple to disassemble to almost cryptically difficult. There are no user serviceable parts inside according to Apple. This should dissuade anyone who is not used to working with delicate electronics from attempting this. This guide is written from memory and may be incomplete. The iMac models vary slightly so YMMV.

Tools required:
• Torx T6
• Torx T8
• Torx T9
• Philips #2, #1 screwdrivers
• Plastic card (credit card or similar)
• Some sort of screw organizer
• Grounding strap.

Disassembly follows:

Remove the RAM door, two Philips #2 captive screws.

Remove three torx T8 screws from bottom of iMac (4 screws on 20")

Loosen front bezel from the rest of the assembly slightly. Do not remove bezel completely.
the top of the bezel is held in my two latch mechanisms that can only be accessed with a plastic card and some luck.
the latches pivot when the card is inserted upward into the rear ventilation slot of the imac at just the right angle.
pushing the card in and forward (toward the front of the computer) makes the latch release and the bezel can be separated from the rest of the assembly with some persuasion.
Each side has to be done separately. This is probably the most difficult part and can take about thirty min of fiddling if you have not done it before.

Once the bezel is loose gently separate it from the chassis and disconnect the microphone cable and camera.
You should now be looking at the bare LCD and a black EMI shield across the bottom.

The black EMI shield is glued into place with tape, separate it from the bottom of the metal chassis along the bottom edge. Be careful it tears easily.
A non marring plastic stick is very useful for this - like this Nylon probe (sometimes local stores like Radio Shack may have something similar)
Similar black EMI tape is around the sides and top edge of the LCD.
Unstick it but do not tear it off or remove it.

Lay the iMac flat on its back. there are four T9 torx screws holding the LCD assembly to the rest of the chassis, one in each corner. They are difficult to see and you may need to bend the plastic sides of the rear bezel out of the way to see them clearly and remove them.
Disconnect the video data cable from the MLB (main logic board), it is held in by two T6 torx screws.
On the 17" model there is also an inverter cable that is plugged into the MLB near the right speaker, unplug it.
On the 20" model you should now be able to tilt the LCD away from the rest of the chassis slightly until you can see the inverter cables that connect the LCD/inverter to the power-supply. Disconnect the bottom two.
Tilt the display away from the chassis to about a 90 degree angle, disconnect the top two inverter cables.
Remove the LCD display and set aside.
You should now have a clear view of all the major components of the iMac.

A Hard Drive replacement is trivial at this point. most Drives are held in via a latch mechanism or two torx T9 screws. Remove the old drive and move the thermal sensor and brackets to new drive and re-install. The ODD (optical disc drive) drive is held in place by an awkward latch mechanism as well. Two plastic tabs on the bottom and top of the drive can be loosened with some persuasion with a flat head screw driver.

To replace the CPU the MLB must be removed. At this point you should decide if you want to continue with disassembly. The connectors for the thermal sensors, speakers and iSight are difficult to remove and easy to break. Look at the placement of the cables, if they are not routed properly they are easy to pinch and cause shorts or other damage.
Memorize or take pictures of the cables you disconnect.
There should be these cables:

• HDD Fan
• Optical Drive fan
• CPU fan
• Optical Drive thermal sensor
• HDD thermal sensor
• Power button
• ALS sensor
• Speaker/iSight/Mic cable
• IR sensor
• Optical Drive flex cable
• HDD SATA cable (this may be UNDER the MLB on some models, we'll get to that)
• power supply cable

Around this time you will also have to remove the speakers (torx T9 screws), the airport/BT card (torx T6 screws), IR assembly (torx T6 screws).
Some cables/connectors hide under the speakers.
You should now be able to see all the Torx T9 screws that hold the MLB in place, depending on the model there are about 7 of them.
Do not remove the screws holding the metal heatsink assembly to the MLB!
Depending on the model there may be two small torx T6 screws holding the top of the heatsink fin assembly to the rear of the chassis, remove them as well. The screws are a mix of coarse thread and machine screws of varying lengths, make you remember where they go.

You should now be ready to separate the MLB from the chassis.
WARNING: the MLB is very delicate. It is an odd shape and has some very thin sections that are easy to flex. Bending or flexing the board will cause the solder joints on the chips to BREAK! This includes VRAM and on-board ram as well as everything else. If you damage the board or the connectors on it you will have a very expensive paper weight! These parts are not available for purchase!
You should be able to pry up on each side of the board until it becomes loose. At that point you should be able to grasp it by the sides (NOT by the metal heatsink assembly) and see if you have any more connectors on the UNDERSIDE of the MLB (the 17" model has the power supply connector underneath). This is extremely difficult to do because the attached cables only have about 1-2 inches of slack. With luck you should have the MLB free of the chassis.

Once the MLB is free take a look at the heat sink assembly. the GPU and CPU have separate heat pipes, cooling fins and are two separate pieces. Locate the CPU, it should have a plastic socket underneath the copper heatsink assembly, the GPU does not.
Remove the 4 torx T9 screws that hold the CPU fin assembly to the GPU half.
Remove the 4 torx T9 screws that hold the copper heatsink to the CPU. The philips #2 screws on top of the heatsink are captive and do not come out.
You should now see the cpu die itself which is removable by twisting the retaining screw/latch with a philips #1 screwdriver or smaller. this screw is plastic and easily damaged. Reassembly is the reverse.
Don't forget to wipe the new CPU and heatsink clean with alcohol and add fresh thermal paste.
When re-installing the MLB it is easy to pinch the cables for the fans and thermal sensors between the bottom of the MLB and the standoffs for the screws. Double check these before tightening anything down.

As you can see this project is not for the faint of heart.

I do not have any pictures to aid in this but there are some visual guides online that may help you:"

http://www.kodawarisan.com/imac_intel/imac_intel01.html (photos of internals)

Jan 2006 iMac Take-Apart Photo Gallery (for HD upgrade) (includes short notes on each photo. I'd forgot to add it to the FAQ's Intel Mac section back then, but I have now.-Mike)

If you have any corrections, additional info, images, etc to add, let me know. Thanks.