I've been syncing again and again over the last couple of days, and iTunes is copying more music over sometimes, though it's inconsistent and unpredictable. Some syncs have copied nothing at all, some have copied several hundred tracks. Storage used for music has gone from 5 to 13 GBs, so at least that's progress, if slow.
I talked with an Apple support rep for a while today, even though I have no official support. I told him that I mainly wanted to make sure that somehow Apple knows that there are lots of people reporting sync issues with iOS 5. When I described what I've experienced and tried, he said the one troubleshooting step left that he knew to try is to uninstall/reinstall iTunes. (See http://support.apple.com/kb/HT1923 for instructions for a full removal.)
Didn't really expect it to make a difference, and after completing my first sync post-reinstall, that did copy 600+ tracks (out of about 8500 remaining that needed copying from the original 11250). But it still stoped far short, so doesn't seem to have made a discernible difference.
I think it would help for as many people as possible to call Apple and complain about this. The main Apple Care line at 800-275-2273 is the number I called.