Heading Elements, Semantics and the Spec
Andy Budd has written a bit about Heading Elements, Semantics and the Spec, noting that there are different schools of using headings.
Here's the comment I posted:
Interesting topic, Andy, and a difficult one. I'm with the "use the highest level heading for the title of the current document" gang. It seems most logical to me. A possible exception is a site's home page, where the site name may be relevant enough to go in a h1 element.
I also don't think putting the site name in an h1 element is going to improve search engine rankings. Think about it: if someone searches for your site/company name, they already know about you, and the search is specific enough that they will find you anyway. You want your highest level headings to describe what your documents are about, so you can be found by those searching for what you are offering.
To make sure search engines pick up your company or site name, you can use the title element to include it on every document if you want to (along with the title of each document, of course).