If you don't get cable cards, you will NOT BE ABLE TO RECORD(*) ANY DIGITAL CHANNELS on the Tivo.
You need cable cards.
Also, I disagree with steve614. Most of the stuff I record on comcast is not protected.
(*) Not in the way that a Tivo is intended to be used. You can manually record channels, but they won't show up with their show name or anything useful.. If you want to do that, use a third party recorder.
Perhaps Comcast is different but the above would be wrong for Time Warner Cable. If you don't have CableCARD(s) you still can tune the analog (SD only) channels and TiVo guide data covers them just as it does digital channels. This is true because TWC (at least my TWC system) has analog-digital simulcast, in which both analog and digital channels are on the cable. They appear at the same 0-99 channel numbers also, although they are actually being tuned at different frequencies. Also copy protection is generally different between analog and digital channels. On my system almost all digital channels are copy protected (no MRV, no TTG) while the analog versions are not.
If you don't have CableCARD(s) and tune "clear QAM" channels, then you have the problem of no TiVo guide data. These channels are provided as digital cable
channels for the local broadcast stations. TiVo's digital tuners will tune them -- CableCARD(s) are not needed, since all they do is decrypt and these channels are clear, i.e., not encrypted. TiVo guide data simply doesn't cover these channels -- there are technical complications involved but not worth trying to explain.
OTA channels (received via the antenna input on the TiVo) will
have TiVo guide data regardless of any other factors. Note that many of these channels will be exactly the same signal as the clear QAM cable channels.