I just stumbled of a neat little feature in iTunes. I'm not sure which versions this applies to other than 7.1.1.
If you are browsing in full-screen Cover Flow mode (View » Cover Flow View, then View » Full Screen) and start watching a video (podcast, movie, music video), you can switch between full-screen Cover Flow mode and full-screen video by pressing Command-Tab, like you would to switch between applications. I ">filmed a little clip of this feature in action, if you'd like to see how it works.
[robg adds: I was quite surprised that this did indeed work -- when in full-screen mode, iTunes overrides the normal Command-Tab functionality. Perhaps this is common knowledge, but it was news to me.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070401090024712