I'm not sure what happened, but I started getting a ton of comments that the new version of iSync supports newer Nokia Series 60 phones like my beloved 6680. I just upgraded this morning, did a scan in iSync and got the message above ("iSync can not connect to this device"). It found my phone, but then gives me an error saying it isn't able to sync.
You know, I freakin' BOUGHT the Mac so these sorts of headaches wouldn't happen. If I wanted to be frustrated by annoying dialog boxes and un-fixable peripheral problems, I would have just stuck with a Windows box.
Update: My whinging worked. The answer appeared in my comments from Kyle Barrow: Delete the 6680 from the Bluetooth device list, then run "Set up a Bluetooth Device..." As you step through the wizard for your 6680, the iSync option becomes available.
Awesome! Thanks Kyle!