It's just happened again. I can confirm the system is 100% unresponsive when it happens. It doesn't respond to ping, there's no sound and an external display doesn't do a thing. Whatever the issue is, it kills the system completely.

This is running an install of Mac OS X Tiger now too, so I doubt it's software related.

Looks like the Radeon Mobility (or the logic board, Branes on IRC suggested the issue could be in the RAMDAC) is on its way out.