I've been hoping that someone was working on using the iPhone browser as a media controller and when I saw Signal, I decided to give it a try.
It works like this: you install the app and it runs an internal webserver that you connect to using safari on the iPhone. Whatever is running in iTunes on your computer shows up on the phone exactly as if it was in the iPod mode (with album art, controls, etc). So if you have an airport express hooked up to a stereo, you point your iTunes at the speakers and start playing some music, then walk away from your computer.
Once you pull up Signal in your phone, you can control your entire iTunes library remotely. You can pick playlists, set them to shuffle, change to specific songs, try different genres, even pull up podcasts and video. The interface on the iPhone is really what sells it -- it looks and functions exactly like a real iPhone app, but it's all in safari. It's really quite amazing and turns the phone into a whole house audio remote for me.