Why would you want such a thing? For starters, JumiMouse could take the place of a laptop touch pad that's worn and no longer working properly.
It could also solve the common problem of accidentally grazing the laptop touch pad with your thumb, causing the cursor to jump and your document to get messed up. Just deactivate the touch pad and use JumiMouse instead.
Finally, JumiMouse is an ideal replacement for netbook touch pads, which are notoriously tiny. The iPhone screen is ginormous in comparison.
JumiMouse communicates with your PC via local or ad-hoc Wi-Fi. To use the app, you must first install the free Jumi Controller utility. It's currently compatible with Windows XP and Vista (32-bit). I tried it with Windows 7: no go.
On a Vista system, JumiMouse was a snap to set up. My iPhone immediately detected the server, and a tap later I was controlling the cursor from my screen. Everything worked perfectly, and I detected no lag between my finger movements and the cursor.
At first I was disappointed by the lack of options for things like pointer speed and size. But it turns out that Windows recognizes JumiMouse as hardware, so all you do is hop into the Control Panel and tweak the standard Mouse settings to your liking.
This is a fabulous little app for anyone looking to control a PC--for any reason--with an iPhone or iPod Touch. Amazingly, JumiMouse is free.