For about a year, I've slowly been migrating to HTML 5. First of course was the DOCTYPE. Next I started using new structure elements such as
<header>, etc. and in some applications - Web Forms 2.0 input types.
To use these elements in Internet Explorer (all currently used versions) and Firefox 2.0 (though I believe this one isn't as important anymore), I wrote a script that looks like a hybrid of John Resig's HTML 5 Shiv and Simon Pieters' Firefox 2 HTML 5 enabling script.
Boy was that fun! I ran into a lot of bugs, and in the end increased my script from about 20 lines to almost 200. Opera was the most tolerant of the new functionality. Firefox had major issues with the DOM, the WebKit browsers had rendering issues, and don't even ask about Internet Explorer. With a few semi-ugly hacks though, I believe I have come up with a forwards compatible solution which I call Fiks.html5.
I plan on adding more HTML 5 functionality as I go. One thing I won't add is support for Web Forms 2 because it has already been done and I believe it belongs in a seperate module.