The template I used as a start for my own design places the metadata inside the header tag of the single-post.php. Is that a correct use of header, or should there only be h-tags in there? What about the post image? Could that be part of the header?
The w3 specification says that only h-tags are allowed inside of the hgroup-tag. The basic purpose of it is to group a main-headline and a sub-headline together.
edit html5 also adds the header-tag to use within the body. Inside of this any other html element is allowed. You just shouldn't confuse it with the head tag. :-)