Bluetooth is great for transmitting data over medium distances and what's more, Arduino just treats it like a serial data connection. This means that we can use the Serial Library. That's a nice thing to have.
But what to pair the bluetooth to? And how do we do that? This covers all that. What you use the new found connective abilities for is up to you.
Here are some ideas though:
Wireless custom remotes.
Pairing with Android for some real power behind it.
Simple alarm system (someone is at the door or in room X)
As always, I made this at TechShop
Step 1: Materials
1. Arduino Uno $40
2. Arduino Shield $10
3. Bluetooth Modem $40-60
Here's the one I'm using from sparkfun