This is built in to iOS 4.3 now.
In iTunes enable "Home Sharing" (Goto Preferences->Sharing->Enable "SHare my library on my local network").
In your iPhone, goto Settings-> iPod and enable your apple id/password in the Home Sharing section.
Now open the iPod app on your iPhone and click More. There should be a "Shared" menu option, which lists your iTunes Libraries. You can stream to your phone and control itunes from your phone..