In previous versions of iTunes, it was nearly impossible to convert a music file into a podcast. Now in iTunes 9, you can now set the type of a track directly to Music, Music Video, Movie, TV Show, Podcast, iTunes U, Audiobook or Voice Memo (not all options are always available).
To change the track type, just do a Get Info (Command-I) on the track, and change the Media Kind field in the Options tab. Changing the track type moves it into the appropriate library, for better media management. Different media kinds also have different capabilities on some iPods (e.g. skip back 30 seconds or 2x play).
Just be sure to make all the other appropriate modifications to the track before changing the Media Kind. For Podcasts, the Remember Playback Position and Skip When Shuffling are checked. These fields are modifiable when the track is Music, but not when the track is Podcast.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090923194952306