This is the interval timer I use on my iPhone. It's a free app that is easy to set and I can still use Pandora while the app is running for my tunes. I set all the intervals to 50 seconds and the rest time to 10 seconds. :)
http://itunes.apple.com/us/app/interval-fitness-timer/id395652793?mt=8
http://itunes.apple.com/us/app/interval-fitness-timer/id395652793?mt=8
