I'm assuming that the following is impossible, otherwise somebody would have done it and be making a mint:

Using bluetooth, have the Treo emulate a headset or hands free device which takes and places calls on other paired bluetooth devices. This would mean that if somebody calls my blackberry, I would be able to use my Treo and pretend that I didn't have a crappy old blackberry ("My first PDA"). Or people could use it with their normal phones (if they had a second phone for whatever reason).

The cool thing would be that you could (assuming that the treo has the processing power to handle it) use the data connection on the treo while routing audio from another device, thus making calls and doing data stuff at the same time. It looks way cooler than holding a blackberry up to your ear while surfing on a treo (Anything looks cooler than holding a blackberry up to your ear.

There must be quite a few people who get given a piece of junk by their work (Not even allowed to get the latest blackberry) and are forced to carry it but would rather be treoing. Palm may not be able to pry the blackberry out of their left hand, but they could definitely add a treo to their right hand.

I read somewhere that the bluetooth profiles are hardware/firmware encoded on the devices, so this might not be possible. But if this is the case, how is USBmodem able to have the treo recognized as a BT DUN modem.