Google has recently released a report on Web Authoring Statistics.
The report, which used over 1 billion documents as its input, analyzes the relative frequency of various HTML elements and attributes. They also mention microformats.org as another initiative which is analyzing markup trends on the web.
The study is worth a read for anyone interested in semantic markup and especially microformats. Beware, however, that in order to see the graphs, you’ll need a browser which can properly render SVG content (FireFox 1.5 seems to work pretty well here).