Well, I can now state authoritatively that the problem was with Mac OS. I still don’t know what went wrong, or why, since I was originally able to sync a few times before it stopped working.
In any case, I upgraded the notebook to Mac OS 10.4 (“Tiger”), and lo and behold! sync worked with the cable right off the bat. And damn is the USB sync cable faster than bluetooth.
So if you’re having problems synching on a Mac, this might be a starting point for diagnosing the problem.