Two possible suggestions: - Sound processing (i.e. more than just images) - Commercials tend to repeat, so you could you this to get another metric for a match. Of course, it wll need a fairly well maintained database...
I am intersted in signal/image processing, so it'd be nice to have a bit of a page spring out from this about the current technical means/ideas for implementing commercial detection.
Do commercials have Teletext/Subtitles? If so could there be a database of Ad scripts?