Although the HTML 5 specification is still in development, on January 22, 2008, the W3C published the first public working draft of the HTML 5 differences from HTML 4. It looks to me as if developers who are familiar with HTML 5 and CSS positioning and styling will have plenty of work upgrading older sites.
No More Frames! According to the working draft, the elements frame, frameset, and noframes are not in HTML 5 because “…their usage affected usability and accessibility for the end user in a negative way.”
What About XHTML 2? XHTML 2, as proposed, is an entirely new language that does not have backward compatibility to HTML 4.01, so the upgrade path will be from HTML 4 to HTML 5.