<< If Push to Talk is Microsoft's proprietary name for phone/PDA convergence, then I have no idea what the specifics are or who has implemented it yet. >>
PTT is not Microsoft's proprietary name for phone/PDA convergence.
PTT is a technology being developed by major networking corporations such as Cisco, Siemens, etc....
It does not utilize typical cellular, GSM, TDMA networks but wireless IP protocols.
Find a palm counterpart, I doubt you will.