There's a funny code you can use to make it call Tivo over IP, something like ,401# or something.It still has to call DirecTV if you use it for PPV, though. (After some limit in the card is reached, anyway)

As to hacking it, search for 'killhdinitrd' over on Deal Database

That's the way in, and once you've done that, it's not much different than a Series2 DTiVo.

EDIT: Here is the thread you want.