Sure. I suggest figure out use scenarios, they might shape how you design it.
How about an interface to Google SMS?
You can send SMS through the phone via Bluetooth, then display the data in a cool, user friendly fashion. Yellow pages, driving directions (less relevant for us with GPS, but still handy), movie showtimes, weather, answers to questions, stock quotes, product prices, dictionary queries, etc. Most of us have unlimited SMS, so why not get a little internet connectivity with a cool lifestyle application to manage it?
This isn't exactly what you were looking for, but it'd be an awful cool use of bluetooth.