Greg Scott from the Windows CE Networking team has posted an interesting article on how Bluetooth Virtual COM ports are implemented on Windows CE.
Alongside the native code to setup a port, there are some details on the improvements in Windows Mobile 5.0 which now includes UI options to set both incoming and outgoing ports and is integrated into the bonding/pairing process. This means that OEMs no longer need to produce their own individual applets to expose this functionality.
For managed code the Bluetooth.NET library wraps this functionality with the BluetoothSerialPort class.