DJV:
I don't think you could make a standalone Tivo work with your built-in DirecTV tuner. They are designed to work with cable boxes or DirecTV boxes. The thorny issue is the mounting of the IR blaster which controls the remote. Also you would have to output you DirecTV signal to the Tivo box, then bring it back to one of your inputs on the TV.
Two ways you can do it, in order of uberness.
1. Buy "DirecTV with Tivo" unit, plug it into your TV as S-video input. You could tape two channels while watching live TV on your TV's built-in DirecTV reciever.
2. Buy Tivo standalone ($300+$13/month) and get an additional DirecTV reciever. Connect these two and run them as an input into the TV. Then you can watch one live channel on the TV's tuner while recording another channel on the Tivo.
I'm not sure how the pricing would work out if you are an existing DirecTV user. For a new user like me, the DirecTV with Tivo box was a no brainer. Free install, and it was cheaper ($150 vs $300) than a standalone Tivo. Plus the monthly fee is only $5 (in addition to DirecTV programming fees.) But they might charge you more than $150 because you are already a customer, so it could tip the scales the other way.
PS If you decide to go with the combo DirecTivo unit, hold off and buy one of the Series 2 units, which should be hitting the stores next week.