I'll be interested to see if Valve will release a Linux-only or a Linux/Windows hybrid box. It is possible today to have a system where you keep two VMs suspended - a linux vm and a windows vm and thaw one to run the program you need, with pci pass-through to give it full control of the video card. This is quite feasible if they go with ATI hardware that has open documentation.

Save system state, boot windows vm, launch game (somehow passing the information to the steam instance in the windows vm, but that's the least of your worries), once game finishes, freeze vm, restore system state and show regular steam-on-linux interface.

I've had before the idea of building a system built on a hypervisor that lets you dynamically switch vms, but the high cost of hardware with an iommu and the pretty astronomical cost of this for a hobby project (i.e. my lazyness) have kept me from doing it.