A few notes.
First, a bit off-topic. The TiVo S3 has six tuners, actually, two each for ATSC, NTSC and QAM. Only two can be active at the same time, though.
Second, regarding Comcast and analog: Generally speaking, if you're still getting any cable networks via analog service, the days for that service are numbered. Analog service for cable networks has ended here already, and my best guess is by the end of next year, it will be a very rare situation -- anywhere in the country, for any service provider, cable or satellite -- for cable networks to be provided via analog service.
With regard to local over-the-air broadcast channels: Analog service for those channels has already ended for just about all of the non-legacy service providers (i.e., Dish Network, DirecTV, FiOS, RCN, U-Verse, etc.) The legacy service providers (i.e., Comcast, Charter, Cox, Cablevision, Time Warner Cable, etc.) have mostly all agreed to continue providing that service until at least 2012, but once 2012 arrives, I would expect to see those service providers phasing analog service out entirely. They're going to need the 150-200 MHz of bandwidth (that limited basic analog service eats up) for advanced services, just to keep up with the non-legacy service providers who have already stopped providing (or never provided) that service.
When, these various changes will happen in your area, I don't know. You can ask on AVS Forum, in the Houston-specific thread. Some folks there might have some idea.