It's thinner, faster, and has built in iSight camera. G5 still, not an Intel chip.
Oh, and there's this:
The new iMac G5 includes Apple's Mighty Mouse, featuring up to four programmable buttons and an ingenious Scroll Ball that lets users scroll in any direction -- vertically, horizontally and even diagonally.
For whoever it was that always bitched that Apples only come with a one-button mouse (yendor?) While the mighty mouse has been out for awhile now, this is the first I've read that it's bundled with a new computer.