I've had an iPhone4 for a week so i'm still very much at the 'kid with a new toy' stage.
Downloaded an app last night which wirelessly connects to your pc and acts as a mouse.
Basically your iphone screen turns into a giant laptop type touchpad, and after installing a small bit of software to the computer you want to control you can use your iphone to control it.
Arrived at work this morning and the first thing I did..... installed the software to a colleague's computer.
When he turned up and logged in the games started!
I've been doing random clicks and mouse movements for the past half an hour, and his confused then irritated then angry facial expressions are priceless!!
The app is Air Mouse Free, and you can download the computer software from: Mobile Mouse for the iPhone, iPod & iPad
And even from a serious side its pretty good. My laptop is connected to my TV and I get fed up of having to keep picking it up to control it. With this app I just use my phone.
Give it a try, if only for the fun of annoying someone.