Apple - What is the use of the Bluetooth on iPad?

You can do peer-to-peer networking on bluetooth with some games and other applications.

I haven't checked out the details but my understanding is that the Bluetooth profile is much better in the iPhone 4 and iPad than in the early iPhones. I expect you could also hook up to BT headphones and speakers to stream music.

In addition to P2P sharing and audio streaming like Doug said, if you have a jailbroken 3G one, you can create a wifi hotspot using MiFi.

Also, some emulators allow you to use a Wii controller via Bluetooth.