Can you think of any reason why placing article credits before the headline, like this:
<p>August 18, 2009 by Author Name</p> <h1>Title of the Post</h1> <p>Post body goes here.</p>
is semantically worse than moving credits after the title:
<h1>Title of the Post</h1> <p>August 18, 2009 by Author Name</p> <p>Post body goes here.</p>
Well, I think it’s not appropriate because H1 suggests that it semantically wraps the content that follows afterwards. Bringing article date and credits before the H1 tags drags them out of context. One may argue that article credits are not part of the post and there is a point in that. But my opinion is that article credits do belong with the post as article without contextual credits is a half baked one (semantically, that is).
It depends on whether you are using HTML 5 in which case you can surround the entire post in an article container. That might go some way to making it less sematically problematic.
Even in that case though a header is a hierachical indicator, so just as an h2 subdivides the article in a way that you can’t break back out of (all content after that point is a sub-section, or sub-sub-section etc) the H1 signals that everything that follows is content under that title.
In most cases the date and attribution, while not part of the article itself, is part of the content that falls under that header so you would expect it to be positioned after the header.
Placing the date and attribution before the header effectively renders it applicable to the content that comes before the header, or to the page in general; although again, you would expect the H1 tag to apply to the page unless using further semantic divisions as is possible in HTML 5.