I have the same problem, just updated my 6210 to a 6310i and have had installed an Advanced HF CARK-91 in the car (CARK-91 installed in to commercial vehicle and 6210 used in the past)
THe 6210 powers up and down when the ignition is switched on, but the 6310 does not react in either the van or the car.
Very frustrating-the manual does not indicate the the ignition sensing option will not work with the 6310i.
Did you ever get to the bottom of this problem? I've got the new Nokia 6100 connected to a CARK-126 car kit and I'm experiencing exactly the same problem.
The first CARK-126 I received didn't work at all so I returned it for a replacement. The new one works perfectly except for the ignition sense. I've checked the IGN lead into the car kit from the car and it's working correctly (zero volts when ignition off, +12v when ignition on).
I don't know if it's a problem with this second car kit or a fault with the handset.
I'm wondering whether the 6100 has a similar problem. I'll be able to test the CARK-126 with a 7210 in a couple of weeks when my brother comes to visit. If the ignition sense works with that then I know it's my 6100. I may even buy another CARK-126 on 7 day approval just to test another unit - if that solves it I'll know it's the car kit.