Logitech's Touch Mouse app recently hit the App Store with the ability to turn your iPhone or iPod touch into a Wi-Fi based wireless trackpad and keyboard for your computer.
The application supports multitouch trackpad gestures, two-finger scrolling, and portrait and landscape orientation. The onscreen keyboard can be hidden when not needed, shows text as you type, and supports modifier keys, too.
Logitech says on their company blog that Touch Mouse requires their free Touch Mouse Server application for Mac OS X 10.4 or higher, or Windows XP and higher.
Touch Mouse for the iPhone and iPod touch is free and available for download at Apple's iTunes-based App Store.