MINI will be releasing a new bluetooth system with armrest at the end of Q1 '05. The system is listed as only working with three phones: the SonyEricsson T610(6), and the Siemens S55/S56, and the Nokia 6310i. Further phones will work with adapters but it's unclear which phones and if they'll be able to take advantage of the bluetooth integration or if they will be able to fit within the armrest. It's also unclear if this will ever make it to the US. It's replacing a previous armrest/phone holder that, as far as I know, was never sold here.
Update: the US will be getting the new version of the steering wheel shown here with the phone controls built-in. It's unclear how (or if) they will work without the armrest.