I've been finding a number of dead entries in the Podcasts menu in Front Row lately. Podcasts that I've listened to and have long since been deleted. Mysterious.
It turns out that there is an enigmatic sequence of events that cause the iTunes database to retain entries for deleted items. I never did find out what are the particular causes for this to occur. However, there is a way to fix these phantoms of podcasts past.
After a few dead ends, I found a thread on the Apple forums about this issue. No one knew why it happens, but one astute poster had a solution. Luckily, Doug has a script that searches the iTunes database and excises incorporeal entries. One run of the script neatly solved the problem. Huzzah!