As a web developer...
... who works on projects with a very tight budget, HTML 5 really won't be an option until about... 2014.
You have two choices, you either support the features that the the stats tell you are realistic (browser usage stats) and get the job done quickly, or you implement new features that older browsers don't support & end up coding in swathes of additional caveats for those older browsers - graceful degradation.
That's all very well, until client X tells you "It doesn't look like the design in the browser we use at work", in which case, graceful degradation goes out the window and you end up turfing out those great new features and relying on the current standards, or hopefully convincing the client to upgrade!
Believe me, I love experimenting with new features, but when the client is breathing down your neck and the boss is counting the hours, you tend to have to stick with the current status quo.
Right now, that's HTML 4 & CSS2, with Jquery to assist in applying at least a few new features.
That's the reality of commercial web development.
When microsoft release another browser, I shudder, as this introduces yet another browser to test on, with it's own set of issues.
For some clients (most specifically those in big corporates), that means having to test in ie6, ie7, ie8 and soon, ie9 - which also means having to support a couple of virtual machines! (and obviously, testing in firefox, safari, opera & chrome - on mac & pc)
In closing, if Paul Rouget is correct in implying ie9 is way behind the pack, it means commercial web developers will, once again, have to wait a very long time to use new features, unless they've got exceptionally wealthy, understanding clients.