Logitech has just announced an iPhone application called Touch Mouse, which lets you use your handset on a Mac or PC as a multi-touch trackpad.
The idea is that when you've plugged your laptop in to your TV, but you can't be bothered to shift that big bowl of popcorn off your belly and get out from under your slanket and change the movie you're watching, you can just whip out your iPhone and use that to queue up the next episode of Battlestar Galactica instead.
The app requires installation of server software on your PC, as well as a download to your iPhone. It supports multi-touch with two or three mouse buttons, two finger scrolling, and also offers a keyboard with ctrl, alt, and command/windows keys. It'll show you what you're typing on the device, as well as on your laptop and both keyboard and mouse work in landscape and portrait modes.
It's free and available now.