One of things we've massively improved in Windows Mobile 5 is it's support for Bluetooth Car kits.
The support implemented allows both handsfree and headset profiles as well as supporting phonebook transfer to a variety of Car Manufacturers Bluetooth Car Kits such as BMW, Acura and many others.
I've personally tried this on my BMW and it transfers all the contacts from my device to the iDrive system I have in my car. I can then call and dial people from the iDrive whilst my phone is in my boot!
This support was added in Adaption Kit Update 1.1 for Windows Mobile 5.
The question I've been getting is... so how can you tell if you are going to get the phonebook transfer and other features. Well if you go into the about screen on your Windows Mobile 5 device then you will see the build number - it needs to be greater than 14402 which means it is using AKU 1.1 and will allow you to get all this functionality.
So far I've tried the K-Jam, SP5, QTEK 9100 and a few other Windows Mobile 5 devices all which appear to work fine with my car kit.
The device that doesn't seem to have AKU 1.1 is the Universal device in it's various guises so I hope the OEMs/Operators will be updating that device soon to use AKU 1.1 :)