Unfortunately Touchpad only works on jailbroken iPhones on firm ware 1.1.4 or less
Lucky there are two other options that work with the current 2.1 firmware Snatch (for Mac only!) or Air Mouse (works on Mac and PC) both are available via the app store or can be errrm... acquired if your iPhone is jailbroken. Note both apps work via network connection Lan or WiFi.
I'm currently using Air Mouse which can be used as both wireless touchpad and keyboard.