I recently stumbled across an MP3 file distributed over RSS (e.g. a Podcast) that isn't included in the iTMS Podcast directory. Dragging the RSS feed URL into the Podcasts Source folder in iTunes subscribes to the feed, and allows you to treat it as though it is.
On closer inspection of this particular feed, it is clearly marked up with Apple's iTunes podcast-1.0 tags, but clicking on the RSS link directly doesn't add the Podcast to iTunes, so I thought this was worth mentioning.
The ACM "Queuecast" feed I'm referencing is available here.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060504214333824