At this point you've gone to the trouble of creating a semantic, user-friendly, and accessible XHTML theme, and one of the benefits of that structure is that it helps with SEO (Search Engine Optimization, if you haven't guessed by now). You might as well go all out and take time to set up a few more optimizations.
WordPress URLs by default are dynamic. This means they are a query string of the index.php page—for example, http://mysite.com/?p=123.
In the past, dynamic URLs were known to break search engine bots that either didn't know what to do when they hit a question mark or ampersand and/or started indexing entire sites as "duplicate content", which lowered page ranking because everything looked like it was coming from the same page (usually the index.php page).
This is no longer the case, at least not with the "big boy" search engines such as Google; but you never know who is searching for you, using what service...