After looking around Amazon's site for a while, and asking by email a couple of times and getting unhelpful canned replies, I asked the friendly customer service people by phone if it was possible to see a list of all the items I'd rated. And it is :-), in 'Your Store' > 'Improve Your Recommendations', which seems obvious now...
That makes much more incentive to go through and give ratings to items, as those pages can be scraped (though not easily, as you have to be logged in) and exported for safekeeping or use elsewhere.