MPEG4 cannot be decoded by the Tivos without major surgery and reprogramming of Linux. I wouldn't be surprised if somebody does this for fun, but I doubt it will be cost-effective for most. Actually, maybe it can't be done at all.

They are already showing commercials for the new DVRs, although just SD models so far. They are probably shipping any time, now. The HD models should be out next spring, although most expect that to be delayed again. MPEG4 basic receivers are shipping any day now, at least to Detroit, the test city.