1) Yes, TiVo can only record what your cable provides you. Current generation TiVo's use CableCard, and will replace your Comcast cable box. Old (2-3 generations ago) TiVo's use an IR blaster to control your cable box and record what it spits out.

2) Only the really, really old TiVo's need a phone line. Anything 6-7 years and newer will be able to do Ethernet.

Bottom line - you'll need a TiVo, a cablebox, a Comcast subscription AND a TiVo subscription.

I'd recommend asking what kind of DVR deal Comcast has for you, or getting a new generation TiVo (like a Premiere) or old Series 3, then replacing your cablebox with TiVo and CableCard (which Comcast will provide you).