I know it's heresy, but it's very possible. This article shows you how to dual-boot a mini, thereby turning it into a BeyondTV box with USB tuner. If Steve won't give us a mini PVR, well by gum we'll make one.
You could also use the EvolutionTV for OS X, but where's the challenge?
SnApple PVR [SnapStream via ZatzNotFunny]