A few of these boxes leaked out into the wider community and recently one of them posted how he made his G5 development system work, with lots of cool pictures. It's possible to turn one of the early 2GHz G5 machines into one of these, using a compatible graphics card (he used a flashed ATI Radeon X800 XT, but it seems easier to just use the native Mac version) and a compatible Intel network card, and a proper selection of ATA devices. There's even a term for these; they call them FrankenXenons. (A reassurance: you won't need to modify your Mac substantially to run this, and you don't need to flash the G5's firmware. On the other hand, this also means that an Alpha can be recreated or even faked with off-the-shelf hardware, and you probably shouldn't drop a lot of coin trying to get a "genuine" one unless you can verify its provenance.)
Note that this probably won't make your Mac into an Xbox 360, or at least not sufficiently to replace a proper console. Many things don't run, particularly later games, and even though it has an original Xbox emulator some of those games still glitch. But at least some early games will run and the recovery image includes a few demo applications to play with.
Eventually the Alphas were replaced with actual pre-production console hardware, called Beta, which have a strange history as Microsoft actually rounded them up, intentionally fouled them to prevent reuse, and shipped them overseas for destruction. Naturally some folks saved a few, as is what usually happens when someone realizes a prototype with potential future value is in their possession, but they have a reputation for being difficult machines.
Even as an Xbox 360 fan, I don't feel highly compelled to seek out the proper hardware to turn an otherwise perfectly useful G5 into a doorstop that runs a few early games rather badly. It certainly doesn't argue much in support for the power of the
dark Microsoft side of the (X-)force, and my slim 360 is much more convenient. But as a historical artifact it's cool that you can do so, if you want to.