There are two options to achieve this, one is jailbreak your iphone
and install "backgrounder". The second is the ooTunes server which
uses apple's safari browser (which can be backgrounded) to play
streams in the background, while streaming them from the server. For
a demo of how this works see the "demos/howto's" link in the
"websites" playlist inside the ooTunes Radio app, or see more info on
this here:
http://support.ootunes.com/forums/40274/entries/29689
Let me know if that helps,
Steve
ooTunes nerd