I have a S3 and it pairs without problems on the Professional Head Unit.
On your SII:
On your BMW:
- Turn BT on on your system settings in Android
- Check the box to make the phone discoverable
Now back to the S2:
- Hit the Phone Key on your steering wheel
- Your headunit will show something like "Please pair Telephone in BT menu"
- Hit the button under PAIR
And back to the BMW:
- At this point your S2 will prompt you for a pairing request, it will show up as "BMWXXXX"
- Accept the pairing request on the S2
And back to the S2:
- Now the display on the BMW Head unit changed ... hit the ADD button
- On the head unit type in a 4 digit security PIN. Hit OK
- The head unit will show "Bluetooth pairing active"
- No look at your S2 and enter the same PIN you entered in your head unit and push OK/Pair
- The head unit will tell you when the pairing process will be completed. When you see the message hit OK.
Now you can use your SII for BT phone calls. The head unit will take a few minutes to upload your phone book. The pairing will not support BT audio streaming unless you have a iDrive system ... but with that the steps above would not apply anyway.
To place a call, hit the phone button on the steering wheel.
Hope that helps.