I believe CNet's writer may find himself eating the words I've emphasized, below:

24-inch iMac not as 3D-upgradable as you might think - Alpha Blog - alpha.cnet.com:
So, sorry, would-be iMac 3D chip upgraders. You might be able to swap out MXM chips at home someday, but not now, and not on this iMac.

If iMac users got Voodoo 2s for an unsupported, appendix-like Mezzanine slot for the first batch of iMacs (wonder if it's related to this?), smart money's probably on some niche company putting out an upgrade for what is an industry standard slot, regardless of how difficult it is to get to.

I've wondered about cutting a hole in the case of my iBook to upgrade the hard drive. When I took my last iBook's case off to replace the hard drive, well, things didn't go as smoothly as I would have liked. Adding an easily accessible door to the general area of the card, though I wonder if it's covered cleverly by the monitor in this case, would be a neat hack.