HTML5 isn't a finished recommendation yet, which means it doesn't exist. You can't write anything that is HTML5. You can write something that adheres to the current draft, which may be invalid a couple of hours from now.
For a more pragmatic reply: you can use some of the new element types that may become part of HTML5 eventually and that are currently implemented in some fashion in certain browsers. As long as you don't rely on those features, since far from every browser implements them.
Normal progressive enhancement in other words. With the added risk that your code may be non-compliant and stop working tomorrow or next year.