More precisely, Steven's site contains a comparative table that shows the current accessibility support to HTML5 elements in the most used web browsers. If you take a look at the table, you will probably notice that almost all HTML5 elements listed there are not supported by browsers from an accessibility point of view. Although these elements are actually implemented, there are no additional accessibility features attached to them. For example, the video and audio elements lack of support to certain keyboard shortcuts in all browsers.
If we want to learn a lesson from this, we should stick to traditional HTML 4 and XHTML elements as long as browsers extend their support to the accessibility features required to make HTML5 elements fully accessible even to assistive technologies. Bear in mind, however, that HTML 4 elements are perfectly valid HTML5 elements, so there are no problems with this approach.