This is what I was told when I requested a refund. I have only had the need to make one refund request, so I can't say whether it is true.

The most interesting thing was that they don't do anything to take the app back. I received a refund, but still have the app on my phone. In fact I think it's even been updated since without a problem.

Here's a snippet from the email from Apple support.

"I have reversed the charge for the accidental purchase you reported. Please note that the iTunes Store Terms of Sale state that all sales are final, so this is a one-time exception."