Finally out of beta, you no longer need an invite to use the free Google Music service. For now, it's available to anyone in the U.S. with a Google account.
Right arrow: Next song
Left arrow: Previous song
Delete: Delete the song
Using Google Music on your mobile device
iOS 4.0+ devices can access Google Music by pointing the browser to music.google.com. Sharon Vaknin has more on how to use Google Music with iOS devices.