When an account is deleted does it retain any personal information about the user so that a later recovery be possible (some sort of logic delete like for deleted answers) or is it a physical (permanent) deletion?
We maintain a record of two users in the case of a merge, in order to unroll/reverse the merge if it was in error, though this is a relatively new addition to things - a little over a month ago.
On deletion (or destruction - user deletion + deleting posts, almost always reserved for spammers) we nuke everything about the user, no record just a line somewhere with a user ID in it, we don't even know what your display name was, much less any private data (email, real name, etc.)
We've talked about recording this on delete/destroy in case a user wants to come back, but we've punted on it until now for 2 reasons:
That's not to say we won't do it to make even the exceptions easier on us, at some point it's likely (just as merges finally became a necessity) the pain of manually reversing both of these is a nightmare, trust me.
If you want to be deleted, that's fine by us - in most cases we're sorry to see you go, but we'll respect your decision. However, when you ask for deletion do understand that you're being deleted, and getting any data back is not always possible - and even if it is, it's a total pain in the ass for us, and time much better spent on improving the site for everyone.
TL;DR version: don't expect your data to be restored (posts associated, etc.) if you come back later, we simply don't have the resources to keep doing that.
My understanding is that deleted accounts are merely disassociated from their posts and other database records. Recovery is difficult, since all of the questions, answers, votes, comments, and so forth must be re-associated with the deleted account. But it is possible.
Accounts with more than 500 reputation cannot be deleted by a moderator. In practice, accounts are not generally deleted unless the account holder asks for deletion under the terms of the CC:Wiki license, or the account is very new and the account holder is already being abusive, having made no effort to understand or conform to what the site is about.