For in car calling I use a bluetooth nokia device. No need for something special here. BUT: charging and having a good view at the phone is another matter.
I asked Brodit for an active holder, this was their reply:



Q: Can you inform me if and when Brodit will release an active holder for the Nokia N900?

A: For the moment we don’t have any plans for this but it can be changed so please check our web for updates in the middle of December.



So let's all mail them to produce a n900 active holder