I agree with all of the above - in American English they are completely interchangeable.

Note the difference in responses from John and James. In British English, you would not often see the past simple used to describe general past experiences (e.g. 'Did you ever go to Australia?'). You would use it for a past action in a specific timeframe as in John's example 'Did you see her new outfit (this morning)?'. In American English, it seems the two forms are much more interchangeable.

These are the interrogative counterparts to the perhaps more familiar declarative forms "you have seen" (perfect) and "you saw" (simple past), and they have the same aspectual connotations. So: a) "Have you seen the Rolling Stones?" (That is, ever, with the opportunity continuing up until now to do so.) b) "Did you see the Rolling Stones while you were in Cuba?" (That is, in some time window that is entirely past.) c) "Did you see the Beatles?" (While you had the opportunity to do so in the past, while they were still performing.)

There really isn't a big difference. "Have you seen her new outfit?" or "did you see her new outfit?" The difference isn't that big, it's as close as trying to dispel the difference between the words this and that. :)