Switch to Other Album Songs While Playing
One feature that I didn't discover until three weeks after buying my iPhone was that you can tap the title bar of a song playing (or tap the "i" button when playing in Coverflow/landscape mode) to display a list of the songs on that album. This is a great feature if you shuffle your music when playing or use playlists because it lets you see the songs and switch to one of them quickly. There's no need to go back and browse. As someone who tends to shuffle and play by playlist all the time, this feature has become one of my favorite little-known iPhone gems. You can also use this display to adjust the ratings of the current song.