It looks like someone has hacked the Apple TV to run Mac OS X along with the full instructions. I got a feeling these instructions or the hacked kernel won't stay there for long, so hurry while it lasts. You will need to supply your own display and plug in a USB hub for keyboard and mouse. Now you can actually buy a brand-new Mac for $299!
The only issue I see with it is whether you can upgrade the RAM beyond 256 MBs. The RAM is soldered onto the motherboard, so you'll have to void the warranty if you want to replace the memory. I won't be surprised if someone tries to upgrade the RAM.