I haven't gotten my iPad yet (9 more days), but I use pandora on my computer and my roku box on the tv. It's a great setup and as mentioned above, has an available app.
You don't need an app if it's streaming. Then there are Yahoo and AOL (apps) with lots of choices to meet mosr peoples tastes and other apps too. Check out the app store in entertainment category but also just bookmark streaming stations in Safari. Try the radio.com app also. Free