Since the last batch of search engine friendly WordPress theme recommendations went over so well, here are five more themes for the full version of WordPress (and if you’re on WordPress.com, don’t despair; we’re in the process of reviewing the included themes right now and will publish our recommendations for you soon).
XV – Revised
We like this theme’s code because it:

validates
has better-than-average code structure
emphasizes your post titles
emphasizes your blog name
emphasizes your blog’s description
uses unique page titles