there are two options:
- the new Arduino Bluetooth, with the module built-in. http://www.tinker.it/en/Products/ArduinoBT
You can get this from PCB Europe ( see hte Arudnio "buy" page, and soon from Sparkfun, I would imagine.
- Using the BlueSmirf from Sparkfun with a regular Arduino.
Look in the "playground" wiki for more info.
We just received 36 modules of the integrated ArduinoBT at the University where I teach, and I can report that they work really well. They are amazing for wearable computing projects!