I spent two days testing the new release of iSync to finally be able to get it to work with my Sony-ericsson P800. There was no real way to do it. I could pair phone and Mac, but as I tried to connect, it gave me the same exact problem, "Could not connect to device P800."
Even trying to re-install iSync, master resetting the phone, repairing privileges, pairing and unpairing time and time again the phone and Mac -- all led me nowhere.
Then I thought of how Apple managed to sync over BlueTooth Symbian phones. Thanks to Jonas Salling who told me that they could only use PPP to sync. So I tried disabling the built-in firewall and ... WOW! It works! With further tests, I discovered that to be able to sync P800 (or any other Symbian phone like 3650, 7650), you have to open port 3004, otherwise you won't be able to connect to the phone with iSync.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030605032502359