One of the most frustrating things about iTunes is the fact that it checks for new podcasts even when you don't have an internet connection. After checking for podcasts, it leaves an exclamation mark next to each podcast (as it wasn't able to update that podcast), eliminating the blue dot.
If you open Front Row, however, all unplayed episodes will still feature a blue dot, letting the user know which episodes are new.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2008020209500711