WordPress 3.0 got a new default theme: Twenty Ten. It’s doctype is yet HTML5, but the rest is still old XHTML. This is where TwentyTenFive get’s into. It’s mission is to get Twenty Ten upgraded to HTML5.
This is indeed a real improvement, but still has the same accessibility issues as Twenty Ten, e.g. not enough contrast in colors, few improvements for keyboard users and you get no real information about user’s location within a web page.