DoctorGOGgles: There are some hardware emulators using FPGAs that come close to your idea, for example
MIST Sort of what i was thinking of. I wasn't thinking so much a system-on-a-chip, rather just the most complex parts of the system that are difficult (
or impossible) to emulate, and with an API to pass to the host system for ram and make calls to the video card. I'd see the Emotion Engine, the Gamecube/Wii CPU's, maybe a cell processor for experimentation. Just the way i see it, an emulator aware of the chip would simply make the calls and forwarding to the CPU's on the board, and deal with other emulation as necessary like audio/midi.
And with cases like the Cell processor, if it had a chance to really get experimented with perhaps the scientific community would use it, or drivers to offload jobs it could handle from the OS.