I believe the Curve suffers from the same problem as most handsets running the latest version of OS 4.2 - the bluetooth stack becomes confused as to the state of the modem connection when you try to disconnect. The practical upshot is you need to pull the battery (hard reset the phone) to get bluetooth tethering back. I've seen the same problem with the Pearl when running T-Mobile's latest release of OS 4.2.

So, try pulling the battery to see if you regain the ability to connect.