iTunes Remote for iPhone
iTunes Remote for iPhone is a handy iTunes remote control system comprising a Windows-based server application partnered with an iPhone web-app. It's quite similar to AlloySoft Signal, but has the advantage of being FREE!
I've heard Apple may be releasing an iTunes remote control application with the new iPhone 3G which could make this immediately obsolete, but we shall see...
You can get the installer and instructions at:
itunes-remote-for-iphone - Google Code