This came up yesterday, and I think it's worth discussing in a little more depth.
There isn't currently an account-deletion feature (though I can do it manually on request), but it's going to be needed in the future. The main thing that probably needs to be decided is what exactly to do with all of the user's posts when they delete their account. If the user was prolific and you wipe out all their posts, it can damage a lot of history. But if you leave their posts up (possibly no longer associated with their username), it means that all of the user's content is now basically "orphaned" and they no longer have control of it even though they posted it.
I won't go into too much detail about my own thoughts, but I'm curious to hear what you all think of how deletions (and the deleted user's content) should be handled.