I just wrote a blog post about an external trackpad for the Mac with multi-touch (found here). But wouldn’t it be cool if someone wrote an app that allows you to tether your phone to your laptop and use it as a multi-touch trackpad?
All you would have to do is open the app on your phone and you’d be able to control the mouse on your screen. You can control your lights from your phone and your iTunes from your phone, so why not your mouse? For all i know the app might already be out there. If it is, someone tell me, otherwise, someone write it :).