yeah, net is right. i think you can upgrade ram, but that's about it. if you buy one and you have enough skill, you can probably change the hard drive to a bigger one, but probably not add another. you don't buy an imac for upgradeability though.
With knowledge you can upgrade anything in an iMac. With no knowledge and an instruction manual, you can upgrade the memory. With common sense, you don't need to upgrade anything.
Personally, I wasn't sold on the new design when I first saw it, but with that said, I hated both the PowerMac G5 and the new Aluminium Cinema Displays, yet now I'm plain jealous of anyone in possession of either, or worse, both. The iMac has definitely grew on me more since this morning, it just looks neat