At this point, HTML5 is not ready to be used, which is why pretty much everyone is NOT using it. I recently did a test of a few of the major HTML5 tags, such as the video tag, one that will be most common, and it failed on more than 50% of the browsers I tested it on. I tested this on firefox, IE, Opera, and Safari for the mac and pc. Then on mobile devices including ipad, iphone, and android froyo. You can test the page yourself by clicking this link. Check to see where it works and does not.
So at this point, I would strongly recommend NOT using HTML5 until is becomes more standardized. Its just full of too many holes. As new browsers get released these holes may become larger or smaller so you really need to just pay attention. W3C has said the standards would be ready by 2014 and you currently can ‘trick’ all browsers into working but honestly I am not going to write code for every browser possibility. That is not the point of HTML5 and if that is the case I might as well use a plug in.
Anyone interested in learning html5 should check out: http://www.w3schools.com/html5/default.asp
And anyone interested in learning more about why HTML5 is not ready for prime time should check out this link. This site describes why youtube is continuing to use flash player and not HTML5: http://www.streaminglearningcenter.com/articles/what-is-html5.html