It goes on a per forum basis, so if it hasn't imported the forum where the new post has been made, then it will pick it up. If it's already imported that forum, it will not however.
One thing to note, importing while the forum is open may lead to some problems, not too bad on classic since it's flat files, but it still will be opening/parsing a lot of files during the import which may cause problems for those that are trying to use the open forums.
