How To Play Your Favourite Internet Radio in Background on Your iPad (if Safari Agrees)
This is often possibile indeed, even if iOS 4 — and its multitasking feature — isn’t yet compatible with the iPad.
The trick is to find an Internet radio you can listen inside Safari, activating it… and then going to the next app. Safari will close, but the stream will continue to play.
Just be sure to choose either a radio Safari can play… and that you like, too!