I am a designer whose main marketing strategy is multi browser compatibility. I assure my clients that the site will work even in IE6 (!).
Of late i have been pondering
"To HTML5" is a fairly broad statement. Even if you have the new HTML5 doctype set (the simple ) you don't have to go all out and use every aspect of HTML5, only what is appropriate to your project.
If you are keen to get on board with HTML5, I recommend reading "How to use HTML5 in your client work right now" for examples of how you can use certain aspects of HTML5 with few (if any) drawbacks.
Disclaimer: I am one of the curators of HTML5 Doctor.
From there it boils down to whether or not your project will benefit from the features of HTML5 and if you can afford to implement these features. For example, if all of your IE users also have JavaScript enabled you can use html5shiv to get IE to recognise the new elements, enabling you to use them and style them.
As for the new JS APIs and CSS properties that people often group with the term "HTML5", unless your site absolutely requires that you use the technology (perhaps something like geolocation), then it could simply be a matter of progressive enhancement. If webkit/firefox users get rounded corners from CSS3 and IE users don't, is that really such a big deal?