I know that the update claims not to be necessary for the unlocked GSM Treo 650, but this page (http://kb.palmone.com/SRVS/CGI-BIN/W...tem,CASE=32400)
on the PalmOne website tells you how to see if the CarKit update has installed successfully, by looking at the size of the Bluetooth library file. On my (1.23) 650, this library file is the smaller of the two sizes listed on that page, which makes me suspect that this patch hasn't actually been applied to the GSM firmware.

I have a Nokia CK-1W car kit, which is not officially supported. The Treo pairs fine with it, and whenever I start the car, the Treo and car kit pair for 15-20 seconds, at which point the connection drops. If I press the car kit button to tell it to reconnect, the connection gets re-established for another 15-20 seconds. As others have suggested, it looks as if the Treo drops the connection if a call isn't currently in progress - does the Car Kit update fix this? In which case I may try installing it.