l337" thing to do: bluejacking. From the website bluejackq.com:
...using a phone with Bluetooth, you can create a phonebook contact and write a message, eg. 'Hello, you've been bluejacked', in the 'Name' field. Then you can search for other phones with Bluetooth and send that phonebook contact to them. On their phone, a message will popup saying "'Hello, you've been bluejacked' has just been received by Bluetooth" or something along those lines. For most 'victims' they will have no idea as to how the message appeared on their phone.
Also check out this rather neat bluejacking account.