It looks like the Visual Web Developer team solved the problem by adding HTML5 support themselves:
http://blogs.msdn.com/webdevtools/archive/2009/11/18/html-5-intellisense-and-validation-schema-for-visual-studio-2008-and-visual-web-developer.aspx
You all probably know that new HTML 5 standard is coming. We made a new intellisense schema that you can add to VS 2008 or VWD Express 2008 and get intellisense and validation on HTML 5 elements.