Given that THREE of the responses to this question are Zappos, I'm going to venture that it's probably incredibly rare. Especially since, in Zappos case, they are essentially selling commodity products (branded clothes that you can buy from any retailer), so their only opportunity for differentiation is price or service.
Zappos has given me one of my best customer service experiences ever. $135 sandals, pit-lab puppy chewed one of them (only a chunk, still wearable). I emailed them asking if I could buy one sandal... they actually sent me a new pair, told me to keep the old ones, and even upgraded me to their premium class where it's free one-day shipping. I never hesitate to recommend them to everyone.
Zappos Anecdote: I reported a bug with handling of email addresses with a '+' in them (ended up in a URL in the email still as a plus instead of URL encoded). I gave a technical description of the issue. I got a prompt response, a notification that it was fixed, and my account was bumped up to some sort of 'premium' status (which I can't use because I'm no longer living in the US, but that's not their fault).