This probably applies to the iPod touch, too, but I've only tested it on the iPhone. If you use the iPhone's headphones (or a third-party set with click-control), iPhone 2.1 adds one more feature to the clicker: triple-click to jump back one song. So one click will pause the current song, two clicks will jump forward one track, and three (very quick) clicks will jump backward one song.
Thanks to Rob Randtoul (creator of some of my favorite desktop images) for pointing this one out to me.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080915055931585