If you've been adding movies to iTunes, you'll notice a lot of the movies show up as a black square or simarly useless preview.
If you've got QuickTime Pro, hit Command-R to reveal the file in the Finder, open it in QuickTime, drag the slider to a more visually appealing frame and select "Set poster frame" from the movie menu. Save the movie and return to iTunes, and you should now see your poster frame and not the black square.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050609083004316