Currently, when a user deletes his account his answers are preserved and show up with an anonymous username like "user3624".
But as tcrosley points out you can't search for that user in the user tab, nor can you do text searches like "user:3624 audio" if you're looking for answers regarding audio by a user who left and was an audio expert.
I think it would be useful to have his account replaced by a dummy, just showing "user3624" instead of his real name, which shows his answers, and would allow filtered text searches as well.
Despite the user's leaving I don't think he would want to deny us his answers.