this is a bit of a silly offering, but it looks cool.
My Nokia phone has Salling Clicker installed, which uses the bluetooth connection to control applescripts on OS X. Applescripts in turn automate system and application funtions, like turn the volume down in iTunes, or put the computer to sleep. You can use the knob on the mobile to control the mouse.
Anyway, I uploaded a command to the phone which sends a binary byte to a perl script, which in turn sends the byte over the serial port to arduino, and turns an LED on or off.
I told you it was frivolous! Despite that, I think it has some interesting potential. The most exciting thing is perhaps to make replace Salling Clicker with a Mobile Processing midlet, and use the PC as a server to communicate with 1 or more Arduino boards.
So I've included links to these files. There's the pde which uses the serial in example, a perl script (which requires the device::serialport module) and a .scpt file; this is the Applescript used by Salling Clicker.http://homepage.mac.com/tobie/mobile-led.zip