While Apple's new iMac models released yesterday appear nearly identical to their predecessors at first glance, the company has of course made a number of changes to beef up performance of its flagship desktop line.
The folks over at iFixit quickly got their hands on one of the new 21.5-inch models and subjected it to one of their thorough teardown routines. Among the interesting findings:

Graphics board and heat sink from new 21.5-inch iMac
- The AMD graphics chip is located on a separate board from the main logic board, allowing for replacement of the GPU without the need for an entirely new logic board. Apple of course doesn't make the GPU board particularly easy to access, as it is not considered a user-replaceable part, but it can be done.
With a bit of magic, the GPU heat sink detaches from the logic board, exposing the AMD GPU board. You heard that right, folks - you don't have to replace the entire logic board if your GPU explodes from too much l33t gaming. You can just swap out the GPU board for another one.
-The new iMac features the same LG display found in the previous generation.
-The optional secondary solid state drive appears to reside directly underneath the optical drive.
The optional SSD appears to reside beneath the optical drive - that's the only space we could find where something was clearly missing. There's three mounting points under the optical drive that have nothing attached to them in our machine, since this option is only available on 2.7 GHz 21.5" iMacs.
The previous-generation iMacs only supported secondary SSD drives on 27-inch models, with the bay positioned next to the optical drive. The bay may still be in that location on the new 27-inch models, but Apple clearly had to find a different layout for the internal components in the smaller body of the 21.5-inch model in order to fit the drive in.

Separate from iFixit's teardown, reviewers have also begun benchmarking the new iMac models with Macworld having already put the new 3.1 GHz 27-inch model through its Speedmark 6.5 testing suite. Unsurprisingly, the machine was found to be significantly faster than the previous generation.
Our overall system performance test suite, Speedmark 6.5, shows the new system to be 16 percent faster than the previous high-end standard configuration iMac, a 27-inch 2.8GHz Core i5 quad-core model with a 1TB 7,200-rpm hard drive, and ATI Radeon HD 5750 graphics with 1GB of dedicated RAM.
Comparing to other machines, the new iMac clocked at about 10% faster than the standard high-end 15-inch MacBook Pro and about 15% slower than a build-to-order six-core Mac Pro.
Macworld will be subjecting the other three standard-configuration iMac models to the same battery of tests to provide comparison data.
Top Rated Comments
I don't think it would be a surprise to anyone to hear there's a generous mark-up on Apple products. It's for the consumer to decide if that's worth paying for or not. There are market forces after all, if it was too expensive, not enough people would buy them - but that doesn't seem to be a problem they're having at the moment.
Still, they are more than the sum of their parts, other companies are free to pick those parts off the shelf and put together something the same or better than the iMac for less. None of the competitors have impressed me with their attempts so far though. I'd be interested to see the cost of the 27" IPS display as well, doesn't strike me as a bargain bucket component.
21 inch base = Core i5 2400S
21 inch high = Core i5 2500S
21 inch bto = Core i7 2600S
27 inch base = Core i5 2500S
27 inch high = Core i5 2400
27 inch bto = Core i7 2600
Here is a link that compares all processors used: http://ark.intel.com/Compare.aspx?ids=52208,52211,52215,52207,52213
That said, the replaceable GPU is intriguing. The worrying bit is:
I wonder if ThunderBolt is fast enough to allow an external GPU that exceeds the performance of the 6970m? Bus to external GPU. GPU to display via Target Display Mode.
Calm down, no need to be rude, especially given that the guy was right.
The glass panel is held on by magnets - I've probably taken apart 50 iMacs so I would have an idea.
The glass panel has a number of metal pins on metal flanges, glued on around its circumference. The aluminium front housing has holes for these pins with magnets on either side. The magnets on the housing attract the metal pins on the glass panel and keep everything together.
If you don't believe me, find something ferrous in your house and wave it around the edge of an iMac's glass panel, you'll notice it get attracted to the 10 or so magnets hidden behind.
What the guy is suggesting is possible, just very unlikely to happen because the bare LCD is so fragile.