Hi,
Due to the way the user handling is done in the core, allowing someone to use the same email address he already used to register a user account to do a guest checkout would require profound changes and bring out complex situations to solve.
So far we didn't work on that. We've been poundering on it for a while but didn't find it a good enough solution to implement it.
So for now, it's still the same. If you already regsitered, then you need to login and can't guest checkout with the email of the user account.