Every few months some consulting firm trying to (literally) scare up some business will come out with a report about how Bluetooth or WiFi or some other thing is vulnerable to hackers and is a major security risk. Usually they at least come up with some catching name for it, and the latest to come along is Bluesnarfing. It's true, you can use Bluetooth to grab personal data off of somebody else's phone or PDA; sharing data is an inherent part of the Bluetooth protocol. The thing is that this can only be done when your Bluetooth is on AND your device is set to discoverable mode, though there are two Nokia handsets (the 6310 and 8910) which are vulnerable even when they are not in discoverable mode, and if you have one of these you need to get a firmware upgrade to prevent the attack.
While you're probably not going to get Bluesnarfed anytime soon (especially here in the US where so few people even have Bluetooth on their phones), the moral here is that you should only set your Bluetooth phone or PDA to discoverable mode when you need to pair up with another device or squirt a business card to a colleague. Doing that should reduce the likelihood of getting snarfed to just about zero. Leave your phone wide open and, yeah, sooner or later something bad is probably going to be open. But you wouldn't leave the front door to your house unlocked either, would you?