Well the mystery has been solved - it was indeed a bad cord.
I returned to the store, tried another seemingly identical cord, and this time E-TTL worked. The store owners were as mystified as I was as to why a brand-new cord didn't work (my first one).
In any case, all is currently well. My 10d, 550EX, and OCSC2 are all now working in harmony.
Cheers to all who read this thread and responded.
RS - Comments on other boards suggest that contacts sometimes need to be cleaned to ensure proper working ; that might be the case with your cable if it is 5 years old.