In the Podcast section of iTunes, go to the podcast at the top of the podcast list, and then hit Command-] to expand all of the podcasts in the list. After you've done whatever it is you wanted to do, use Command-] to collapse them all again.
[robg adds: This one isn't documented anywhere that I can see. The iTunes team has found multiple uses for the Command-Bracket shortcuts, it seems.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060201144427464