You need to check that both the 3100 and the 5800 are using the latest software. There were problems with the older software but they have been resolved.
On the 5800: Settings>Phone>Phone mgmt.>Device updates>Options>Check for updates (Assumes your 5800 has a working internet connection). Othewise use software updates from Nokia PCsuite.
For the 3100: Download the software update tool from www.parrot.com
to a bluetooth enabled notebook. Run the update tool and just follow the instructions. If you don't have wireless internet on your notebook, run the update tool first while connected to the net to download the update files before taking the notebook into the car. The 3100 must be in software update mode before you connect the notebook via bluetooth. Settings>Advanced settings>Software Update>Activate.
I found that it took a couple of attempts to get the connection to work, but once it was made the update completed fine. The 3100 then resets itself and you need to pair the 5800 again. Once it's paired, the contacts will synchronise.
I did this about 30 minutes ago, so I know it works.