Tonight we tried watching 24 about half an hour after it started airing, thanks to the magic of MythTV. Unfortunately, today's 24 did not appear in the "recorded shows" menu. In fact, no show had been recorded since Thanksgiving. Some debugging revealed that mythfilldatabase hadn't actually updated the program database since 11/19 due to some changes at the site that the data is ripped from, so once I hit the end of the 7-day buffer, no shows were recorded. A new version of XMLTV was released on 11/19 which would have fixed the problem, but silly me, I didn't notice that there was a problem and thus didn't upgrade.

I am now subscribed to the XMLTV project, so I can see when new releases come out and will have a minimum of 7 days to upgrade.