From searching SO, this question was already asked, almost a year ago now.
So now with the new FF, Opera, IE, is it finally time to start developing sites with HTML5 or is it still a little premature and will cause compatibility issues?
Is using HTML5 just going to require us to use more and more JS on websites to ‘trick’ older browsers into working properly?
If you add nice features to your site, it is possible it will be talked about and reach the news sites for some free publicity.
Aside from that, It would make a good beta site and give you a head start for when it becomes the new technology. However, until HTML 5 enabled browsers are widespread (at least 20% of the market, possibly 50%) it makes little sense to alienate nearly the whole internet.