I've never been concerned about this in the past because the RFC states clearly that the client MUST wait for the continuation response before proceeding, so no issue has ever previously arisen as a result.
The reason we have such a high level of interoperability in IMAP is because almost all of us follow the letter of the specification. The RFC is very clear, and the client is very broken. Don't cave in to other peoples laziness. (And this case is trivial to fix in the client.)
--lyndon
