I was watching the most recent Macworld video podcasts where they were talking about using a Mac Mini as a home theater. They mentioned an app in the video called Air Mouse Pro (iTunes link).
Air Mouse Pro is an app which essentially turns your iPhone or iPod Touch into a trackpad-style mouse for your Mac, or PC. The app uses WiFi to connect to your desktop. I had success connecting to my MacBook both through the existing wireless network, and through an AdHoc network, so even if there is no WiFi where you happened to be, you can still use the app. Air Mouse Pro turns the screen of your device into a full function Multi-Touch trackpad. If you are hooking to a Mac then it assumes the same functionality as your portable would have, or limited multi touch with a desktop machine. After you install the menu bar application on the desktop, which is required to use this app, you can fully control your Mac.