The script exists - you can grab it
here. The problem lies with RIM. the most recent versions of the 4.2.1 OS have a bug in the bluetooth stack that results in the phone becoming unable to reconnect after a tethering session is disconnected. The only solution currently available is to pull the battery (hard reset) every time you want to tether.
I have high hopes that OS 4.3 will solve the problem, but we'll have to wait until it's released for the 8300 series before we know for sure.