I double checked my MobileMe account and password - that's not the problem.
The release notes for the 104457 mentioned sync updates, which encouraged me to try again:
- On my iBook (Tiger), I had not turned off syncing. I got one authentication request before updating. After updating, Sync quietly did its thing.
- On my desktop (Leopard), I did the SP update and then reenabled Sync. It got an authentication request (blank fields), which I filled in and checked Remember in Keychain. The Syncing progress bar appeared in the prefs panel. When the progress bar got to 25%, another blank authentication request appeared, which I filled in.
Since then, the desktop Mac did an auto-sync quietly (no authentication request), and I've done a manual Sync Now on both Macs. All quiet for now.