Take, for example, the Xbox Media Center iPhone Remote. If you're still using your old Xbox as a media center, Tom Robinson has devised a setup that will let you actually log in to your Xbox from your iPhone, essentially using the iPhone as a very expensive (but very functional) remote control. Basically, you FTP the code onto your Xbox (or just install it over the web), and then login to the Xbox's IP from MobileSafari, and you can use it to browse media, view information on playing media, and even use transport and audio controls. Wild.
My only question is how secure this is-- without having used it, it seems like all you need to get full access to the Xbox is the IP address. There may be something in the code that confines access to a local network, but if you can install this script from the web, it seems like you could also log in to control an Xbox from afar after it's been installed. At any rate, it's a really interesting idea, and just another example of how creative people are getting in developing functionality on the iPhone.