As usual, our friends at iFixit have taken apart the latest hardware from Apple. Inside the new 20" iMac, they found that the machine's innards live behind the display, which must first be removed (this isn't new, but a lot less convenient than the G5s with their removable rear panel).
Notable is that the AU Optronics M302EW02 display isn't LED backlit, but has a CCFL backlight (four of them in fact, each run by its own high-voltage AC power). Also, the crew had to remove 22 screws before getting at the 12.7mm SATA 8x double-layer SuperDrive, which features a screw-free design. Go figure.
Finally, they uncovered six temperature sensors, the largest resting above above the 2.66 GHz Core 2 Duo processor, of course.
While my first inclination after receiving a brand new iMac certainly wouldn't be to take it apart, I'm glad that's just me. Thanks, iFixit!