"Something you have" - much like this...

The Nokia BT necklace accessory: a remote bluetooth device, maybe in the form of a wear-around-the-neck gadget. Once you install the Nokia software that comes with it, it will effectively disable the need to authenticate if it's within 2 meters range of the neck gadget.

In addition to the lock/unlock functionality, it may or may not also have support for the AVRCP protocol.

Edit: I didn't say it cause it was so obvious to me, but just in case:

If the N900 is not near this bluetooth key, you fall back to the most rigid inbuild authentication method, preferably a password.