Yup, its possible, its easy, infact when im bored at work i do do it as we have tons of PDAs and bluetooth phones here for the engineers.

Jus bloody turn the bluetooth option off unless you paired upto a PDA like Nige said.

You can change the comfermation code (usualy 0000 or 1234) but theres ways of hacking that.

I wouldnt trust it at all.