The original forum post has quite a bit of additional information about poking around inside the new 27" iMac. Bottom line: I'll wait 6 months or so for iFixit or OWC to start carrying "upgrade kits" with the proper replacement tape & entry tools.
The 27" iMac holds one 3.5" drive (unlike the 21", which uses a 2.5" drive), and one "blade" SSD like the new MacBook Pros use. Unlike the new 21" iMac, you can upgrade the RAM on the 27" w/o cutting the machine open (there's an access hatch on the back).
Replacing the disk requires cutting open the display and removing the left speaker. You'll need a proper mounting bracket (e.g., an Icy Dock) to mount a 2.5" SSD in place of the 3.5" drive. Adding/replacing the blade SSD requires pulling out the main logic board.
Creating huge access headaches just to make the edges a 1/4" thinner is just insane.