According to this note on Mobilewhack, Sprint will eventually release the locked bluetooth profiles to allow the Treo 650 to be used as as modem. Score one for consumer protest.
Due to some development deadlines, the phone has been launched as described, without the DUN [dial-up networking] capability. However, as part of a scheduled maintenance release of software (timing pending some testing), the DUN capabilities will be supported. … In no way is Sprint suppressing the functionality as you describe or with the motivations you assert. [MobileWhack]