Yes, I did get iCloud activated... the problem was a setting on my 3GS: Settings, General, Restrictions, Accounts, Allow Changes.
I had previously blocked Account Changes. After changing it to Allow Changes, iCloud activated. So now I can sync the 3GS to iCloud, and 3GS to iTunes on my Windows XP laptop.
What puzzles me is that, when I just now synced the 3GS to iCloud via iTunes, it also worked! Supposedly, Windows XP and iCloud don't work together.
I'm not sure how to verify that the sync actually worked; but there were no error flags, and all Apps, music, & books are still intact.
I guess I'd have to do a Restore from iCloud to check, but for now I'm leaving well-enough alone.