Strange. I never assigned any comm ports to mine and I perform an Activesync via BT all the time. I initiate the AS from my Treo 700wx only and never from the PC, if that matters.
I created a step by step procedure on a forum, but I can't find it at the moment, so I'll try to remember the steps...
First of all, I already had Microsoft Activesync 4.5 installed with Hannip A2DP app.
1. From the Treo, pair your device with your PC
2. You should have an option for Activesync on your Treo, if not proceed anyway.
3. From the Treo, perform an Activesync. If a device without Activesync is not found, it will search for a device that has one. This should then add the AS option for your PC, which you should now see on your Treo. Select it.
EDIT: I found the post I made on SprintUsers.com
here...