That's a lot of high-end hardware MS is putting into the X-Box 360. The ATI card alone would likley retail (if it were separately available for a pc) for around $500. I have a sneaky suspicion that this console is going to cost around $750. Hopefully I'm wrong it it comes in for around $360 (which ties in nicely with it's name).
The package (unless MS sees fit to change it at E3) is:
The XBox 360 console
One wireless controller
One 20GB removable hard drive
As has been noted, without a massive OS and numerous different componants to support, 512MB is a lot. Keep in mind that the GPU (graphics card) will be monopolizing some/most of that.
I find it highly doubtful that any games will come out that fully utilize the six threads that the multi-core CPU can use for at least 2 or 3 years (at least, in any meaningful way).
It is doubtful that you will be able to use the XBox 360 as a computer. It will support audio and video calling/conferencing and even accept streaming media from your pc. But if you mod the XBox, you won't be able to use it for XBox Live... and MS has mandated that all games must use in some fashion XBox Live.
Supposedly there will be a wireless networking card available in the near future; making the box completely wireless (except for the monitor/hdtv/audio connections).
I wouldn't be surprised if MS forgoes backwards compatablility. It's not really needed unless the software available at launch isn't very good.
Gamespot.com has a nice writeup on the XBox 360 that's a pretty interesting read.
As far as the CELL goes... I think programmers are going to have a heck-of-a-time writing for that monstrosity. And while it can do a whole heck of a lot at once, it's not a very advanced chip (not that it needs to be for a console). I think that, given AMD & Intel's demonstrations of dual-core CPUs recently, the XBox 360 will be the most powerful system on the market.
Time to pick up an HDTV, I think.