HTML5 is largely mature enough to start using now with ASP.NET websites.
To start with, get the Web Standards update for Visual Studio 2010 SP1: http://visualstudiogallery.msdn.microsoft.com/a15c3ce9-f58f-42b7-8668-53f6cdc2cd83
The following are a list of things that I do to be compatible with HTML5.
Use the html5 doctype at the top of every page:
<!DOCTYPE html>
Set the lang in the html tag:
<html lang="en">
Declare the character incoding in a meta tag in the head:
<meta charset="utf-8" />
Don’t declare the type or language in script tags:
<script src="/Lib/JavaScript/jquery-1.6.2.min.js"></script>
Don’t declare the type in css links:
<link rel="stylesheet" href="/Lib/Styles/colorbox.css" />
Include this so old versions of IE behave:
<!--[if lt IE 9]>
<script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->
Use <header>, <nav>, <section>, <article> and <footer> tags for the page structure
Get more info at http://diveintohtml5.org/semantics.html
Blaise Alicki 