No Series 3 Tivo yet, but Amazon has specs on a SD dual cable tuner (one analog, one digital). Sounds like they may have one input for an external tuner and one internal tuner. Probably not the most straightforward thing to hook up, but Tivo has been pretty remarkable in the past making this stuff just work.
No official word from Tivo yet, but Amazon, Buy.com and a few others seem to have product info, but no ship date.
(Via Zatz Not Funny. . ..)