As a bit of a side project, I decided to add LED call and message indicators to my purse. Fun and good for noisy places where you might not hear your phone.
I attached four RGB LEDs to a lilypad arduino, and used a Bluetooth Mate shield to connect with my BlackBerry. The Bluetooth Mate acts as a serial port, so I wrote a simple program that drives the LEDs over serial, and put all the indicator logic in an app on the phone, as it has the better battery.
So far, the indicators light up different colors for different types of messages (personal email, work email, SMS, etc.) and all four lights flash for an incoming call.