A really cool sounding application for using the iPhone which I stumbled across the other day was the ability to use it as a fully functioning wireless mouse and keyboard via the Touch Mouse application.
Here is the full description:
Touch Mouse by Logitech turns your iPhone or iPod touch into a wireless trackpad and keyboard for your computer.
Do you connect your computer to your TV? Touch Mouse is the perfect solution for when you want to stay in control from the comfort of your couch. Point, click, scroll and type in any application—on a Mac or a PC.
Touch Mouse works over Wi-Fi, so you can use it anywhere in the house. And to make typing easier, text is displayed on the screen of your iPhone or iPod touch as you type, so you can see what you’re doing without having to continually look up and down.
The second paragraph is where I can see this being most useful. For a HTPC, not having to pull out a keyboard and mouse to use the system can be incredibly convenient for situations where the remote doesn’t easily do what you need.
One of the requirements, however, is that you install the “server” software on each of the machines you want to use this on so that it will accept the commands. Still, this is a minor one time setup which will probably only take a minute or two to install.