Browser support is coming along surprisingly quickly considering HTML5 reached Candidate Recommendation in December 2012 won't reach full Recommendation for some time. However, the reality is that browsers that don't support HTML5 very well still make up a significant share of the market. For example, as of this writing, versions of Internet Explorer prior version 9 still occupy a significant market share. See the chart below:
That said, we can start playing with HTML5 now and even developing full-fledged applications. We just need to be aware of browser limitations and attempt to degrade gracefully.