EDIT: I made a mistaken in the path below. The key name should be HKLM\Software\Microsoft\Windows CE Services only. Underneath that key (in the right-hand pane in Regedit) there should be a String called SerialPort. You need to change the value of that string to COM7 (or whatever your Bluetooth COM port is). Apologies.

If you are running the new Windows Mobile Device Center on Windows Vista, and using the inbuilt Microsoft Bluetooth Stack, then syncing via bluetooth probably works fine for you. However if you are using the Toshiba Bluetooth Stack (for example, to get A2DP profile support) then you need to do some work to get bluetooth synchronising working.

Firstly, on your host machine, open Regedit and navigate to:

HKLM\Software\Microsoft\Windows CE Services\SerialPort

and change the value from Bluetooth to your incoming bluetooth COM port. Normally this is COM7 if you are using v5 of the Toshiba Bluetooth Stack (the Vista compatible version). Change the value to "COM7" (without the quotes). You will need to restart your machine.

Now pair your Windows Mobile device with your host machine. You should see the ActiveSync service listed. If you've previously paired your device, then you would have seen "Serial Port" listed as a service (which isn't enough to get ActiveSync working). Instead go into the properties of the bluetooth device and select "refresh" to get an updated list of services that your host PC is advertising. ActiveSync should now be an option. Select it. Now, when using ActiveSync on your WM device, Connect via Bluetooth should work just fine.