There's a new security program for the Treo 650: Bluekey. I've downloaded the trial version and am playing around with it.

Bluekey will lock your Treo, or just specific applications, if your Bluetooth headset, or other Bluetooth device, is turned off or out of range. Only the specified Bluetooth device can unlock your Treo 650 (or a password, of course.) So if your Treo is stolen, the moment it's out of Bluetooth range your Treo locks up tight. Interesting security application.

Bluekey can be downloaded from Palmgear: