i have a nokia ck-300 bluetooth car kit and my n900 works well with it while its connected but as soon as I get out of the car my phone does all sorts of crazy things including-
* freezing (sometimes to the point that battery needs to be removed),
*when i call people they cant hear me and I cant hear them (phone needs to be turned off and on to fix)
*in coming phone calls in general mode are silent without vibration (phone needs to also be turned off and back on to fix)

I took the phone back to vodafone a week after buying it and they gave me a brand new n900 in the box and the new phone does exactly the same. With bluetooth off the phone is problem free.