Props to the power of the Internet and fast, high-quality customer service...
All of a sudden one day last week my Tivo stopped changing channels on my cable box, which was annoying while watching live TV and caused the Tivo to record the wrong programs the rest of the time. I double-checked the Tivo settings just to make sure nothing had changed (it hadn't), then lived with it for a few days because I dreaded having to crawl behind my entertainment center after a long day at work.
Tonight I finally got around to looking up the problem on the Internet, and quickly came across a post on the Tivo forums that described the problem was due to a Comcast update that disabled the serial port on the cable box. I called up Comcast customer service as suggested in the forum, and within two minutes the problem was fixed. I was even able to verify that channel changing worked again while still on the phone with the cust serv rep.
Thanks Tivo users! Thanks Comcast!