My gut feeling is that the mobile arena might be where html5 takes off first, with the number of webkit-based browsers out there now (iphone, android).
How does that affect you? Your HTML markup will work in HTML5, too. In any case, I keep reading no one will use it for years yet most of the leading developers and designers I know are using it today. And I'm joining them since all my markup is now HTML5 as of six months ago.
When you say that your markup is HTML5, do you just mean that you changed the doctype at the top (That's trivial, and its only real effect is saving a few bytes, though I suppose it's a nice symbolic gesture), or that you are now using all of the new semantic elements like <article> and <section> and <footer>, using <video> instead of plugin-based video, including features like drag-and-drop, or offline use of data-heavy apps, &c. &c.?
Hopefully many of the people on IE 6 will leapfrog to a HTML 5 browser in the next few years, that would solve a lot of issues.