From time to time, those machines reappear on eBay or at private homes – I have one – but obviously without the Microsoft OS (and with Mac OS X).
Then, Microsoft downgraded the machines, which were eventually sold to employees for $ 300, with obviously a completely erased hard drive. The development kits are used during a few years, at least until the E3 2005: the demonstrations of the Xbox 360 at the time are on Mac. As we will see, they differ essentially by the graphics card used.
Before the arrival of the real console, the brand provides two variants to the developers: the Alpha 1 and the Alpha 2 kits. Then, Microsoft buys many Power Mac G5 and installs a dedicated operating system on it for the devs.