If a post has already been saved, using the inline uploader works as expected: the uploaded file is associated with the post. (You can tell it has worked, because the file appears in the uploader's "Browse" tab when editing the post, that is, when viewing a post with /wp-admin/post.php, under the "Manage" tab.)
But using the inline uploader before it has been saved (that is, when editing it with /wp-admin/post-new.php, displayed under the "Write" tab), uploaded files DO NOT get associated with the post, although they do get uploaded. (There is no "Browse" tab in the uploader, just a "Browse All.")
This is unexpected behavior and a change from WP 2.0.x. It's a significant problem anytime you need uploads and posts to be associated with one another (as many image plugins do, for instance). Naturally, the work around is to save first before uploading, but even so simple a work-around as that shouldn't be necessary, imho.