Here is a bug in the current version of iTunes 7.4.2 and the earlier 7.4 versions. I have an iPhone and see this. It will likely appear on video iPods as well. Videos watched to the end and deleted to conserve space will not be marked as viewed in the next sync. Here are the details.
When you watch a video podcast to the end, a dialog pops up asking if you want to delete the video to conserve space. If you agree to delete it and then sync the iPhone, the video is not marked as watched in iTunes. So, if you have a preference like “Sync 10 most recent unplayed episodes of all podcasts” set, it will not work – the video you deleted is not marked as played, and in the sync, it is simply copied back onto the iPhone in an unplayed state.
Earlier versions properly marked such deleted videos as played, and they would not be copied back.
Note that if you choose to keep the played video, when you sync, it will properly be marked as played in iTunes, and with the preference set as above, will correctly be deleted from the iPhone.
Do you also see this bug with your iPhone or video iPod?