Okay, so I made a small java app that can take accelerometer data from the phone, send it via wifi to a computer and control the mouse.

Step by step for nokia 5800:
1: Install sensor_s60_5_0_beta.sisx on your phone (will add JSR 256 (Mobile sensor API) support)
2: Install phone_mouse.jar on your phone
3: Run the pmserver.jar on your desktop
4: Run PMClient on your phone
5: Have fun! (for like half a minute)

Please try it out and bombard me with improvement suggestions =D
And remember, this is extremely early testing version. Mouse click functionality will come eventually.

Edit: New server app, a little easier to run and handle.

Download Here