While reading an interesting article on how Google grows, I stopped on this:
A page that displays a search word in boldface or in the upper-right-hand corner, for example, will likely rank higher than a page with the same words displayed less prominently.
Having spent a good part of the day reviewing a CMS and talking about semantic and presentation, I was thinking that XHTML should make Google's life quite easy in that they would have a great deal of the relevance job already done by how the content is semantically organized, without the need to interpret the presentation. If you compare how much sense a <h1>...</h1> header or an <em> makes compared to "boldface or in the upper-right-hand corner", it's easy to infer that Google should return much more relevant results on sites which content adheres to the semantic spirit of web standards.
If this hypothesis holds true, the W3C should talk to Google and get them communicate on that. It's a true win-win situation. The more semantic content is published, the less ressources Google has to throw out to index it. And if Google starts telling the world that web standards compliant sites rank better than the "old school" ones, imagine the boost for the W3C standards!