After PS3 release, a few comments are needed about hacking next gen consoles...

If you need 2D homebrew, you have unmodded PS3 now
If you need 3D homebrew, you have XNA interpreter on xbox 360 (you pay, players pay, so you can't do any business, but it's not like you really planned to do some...)
(it's odd, it looks like a complementary association in order to bring hacking pressure down)

Eh... that seriously lowers hacking motivations (without talking about huge security obstacles of next gen).
(piracy already exists, no more pressure from that part)

The only last interesting thing I see would be to have the pleasure to directly have low level control on one of these last generation GPU. Xbox 360 GPU seems to be the winner in front of PS3 GPU. And xbox 360 hardware seems more easy to manipulate (one board) than PS3 hardware (very tricky encapsulated hardware parts).

Let's say you put the xbox 360 board vertically in a new tower case, and keep enough room on both sides of main board. Then you solder a CPU support on other side of main board and create a switch that either brings power supply to standard xbox 360 CPU or to the alternate CPU you can plug in on other side of main board. To make the soldering possible, we can imagine a support with short metal pins with some soldering matter ready on them and ceramic parts around pins that prevents the pins to go too deep inside main board and have the soldering made through a global heating (like the hotgun repair method).

On the support, you can plug an intermediate "shuffle" support that will redirect lower pins to correct upper pins plugs. Each "shuffle" support matches a replacement CPU able to run with xbox 360 hardware.

And then hacking becomes easier : the incredible CPU security gets out of the way.

Task can be devided in 3 parts, done by different specialized people :

a) "straight" support soldering business (delicate, and hand made for sure)
b) "shuffle" supports manufacture and choice of associated replacement CPU
c) creation of operating system for the replacement CPUs (let's say linux type at start)

Then starts the joy to discover how GPU works...

Unless...
Do you think data sent to xbox 360 GPU is encrypted?

I'm not skilled enough to attempt a) or b) preliminary steps though...

Im not exactly sure, and cant test myself, but I would assume MS did everything possible (in there minds) to prevent hacking, so I would guess that they encrypted as much data as possible that is transferred in the Xbox 360.

I'd have to guess that little to no data going in and out of the GPU is encrypted. You can tell how important memory read/write speed was to microsoft by looking at the cpu/memory diagram

And the cpu your going to use is?????

Would that work on a multiple layer PCB?