Including the logo image in an actual img tag with an alt attribute (like the way it is here currently) would be most appropriate, as opposed to displaying it as a background-image via CSS. This way search engines can index your logo, and it would turn up in image searches too.
Trust me, bear Berners-Lee quirky presentation style and see how a "semantic web" can revolutionize our notions of the internet.
Even though he is talking about "linked data" on a macro level, on a micro-scale, "semantic" HTML can make the content on your community re-usable, malleable, and mash-able (CMS ? ) using the advantages of (X)HTML as a semantic mark-up language.
Look at most of the new HTML5 tags if you want to see what symantic HTML is all about. It's about telling the browser/user/reader which parts of the page are content and which parts are navigation, user profile info, sidebar, etc.
Two years ago, we worried about Google indexing duplicate content from page-to-page. (t=49213, p=95382 taking you to the same page).
Today, we worry about Google indexing duplicate content within the page (navbar, user profile fields, header, footer, breadcrumb, user signatures, etc.).
It's not enough to do a Google sitemap and avoid duplicate content. Now we map out each page to help Google rank content.
Semantically correct HTML in essence is about using the correct tag for the correct job, correctly nested, correctly identified throughout, so the reader can completely understand the structure, etc etc...
It is semantically incorrect to be using font tags in xhtml, or using italic tags vs. the semantically correct emphasis tag.
The semantically correct code for HTML5 will be the use of the header, footer, nav, etc tags, not div with an id, even though both could be argued as semantically correct for backwards compatibility.