I've heard it said (though admittedly only by one person) that in the really top-notch sushi restaurants you eat with your fingers, not with chopsticks. This sounds a little unlikely to me, can anyone confirm or deny it ?

Not first-hand. However, the person I know who is far and away the most expert sushi eater is dracunculus. She mentioned this issue in a post a while ago (which I won't attempt to summarise apart from to note that it's not a total myth).