I had the same problem. I found these:
http://discussions.apple.com/thread.jspa?threadID=2045376
http://discussions.apple.com/thread.jspa?threadID=1957042
What I did:
- Remove the Mac from the iPhone bluetooth pane (forget this device).
- Remove the phone from the bluetooth preference pane.
- Remove Bluetooth PAN from the network preference pane.
- Remove com.apple.Bluetooth.plist from /Library/Preferences/
- Reboot the Mac.
- Pair the phone through bluetoote preference pane.
- Create Bluetooth PAN in network preference pane.
- Connect the network from the bluetooth preference pane.
Now, it stays connected: posting this via tehering over bluetooth.