If any of the tags in an MP3 contain the text "podcast," then iTunes will treat that MP3 as a podcast. The problem is that iTunes won't let you edit all the tags that may contain this text, in particular, the subtitle tag (the "TIT3" frame from the ID3v2 spec), which is what you see in the "Description" column of the Podcast screen.
Unfortunately, I haven't been able to find any apps that allow you to edit arbitrary tags, including non-standard ones, without resorting to Fink or Darwin Ports. (But I'll bet someone is will point one out in the comments.) I ended up downloading and compiling id3lib and id3v2 to get at all the tags.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050917182133400