I doubt the HD-DirecTiVo will support MPEG4. The zillion dollar question is what the migration strategy will be from the HD-TiVo to the in-house HD box. My bet is that the current HD-TiVo continues to work with the channels that it receives today for several years. The new satellites that are going up will transmit MPEG4 but the old satellites will continue to broadcast their current (non) standard signals.

Why no immediate cut-over? Consider that they have a massive installed base of SD receivers that they clearly cannot change overnight. Further, their HD customers (TiVo or otherwise) have generally invested big bucks to be early adopters. If they have any marketing brains, they'd rather offer incentives (i.e., new channels, new features) to make me want to migrate, rather than telling me that my $1000 box is obsolete and I must migrate.

Which means, if you're satisfied with the current small selection of HD channels (HD-HBO, HD-Showtime, etc.) plus whatever HD you can pick up with an OTA antenna, then the current HD TiVo should be safe to purchase. If you want HD locals and you need them from the satellite (i.e., your OTA reception sucks), or you want TiVo features like HMO that they're only promising on the "new" in-house HD boxes, then you're going to want the new, as-yet-vaporware solution. Since my OTA reception is pretty good and I can live without HMO, I'm okay with where I'm at.