I used to use Salling Clicker with my Treo and my Sony Ericsson for remote control over Bluetooth. It was really cool that my Mac would recognize when my phone was nearby, and would start syncing or do other stuff that I thought handy. The iPhone doesn't do very much with Bluetooth, but this developer has figured out something pretty useful. Not bad for $8.http://themha.com/airlock/
Airlock allows your Mac to lock itself, plain and simple. Using your iPhone or iPod Touch, Bluetooth, and a smidgen of pixie dust, Airlock determines whether you're near your computer. When you leave the room – poof! – your Mac locks itself. “And when I come back?” You guessed it: your Mac unlocks. You can also customize Airlock to perform specific actions as you come and go – have your computer talk to you, log-in or out of iChat, walk the dog, and such.
(By the way, this is yet another tip I got from the picks by the guys on the MacBreak Weekly podcast (iTunes link). They're always mentioning useful stuff, and they maintain a nicely rounded perspective on the Mac, while still being obvious fans.)