There's no reason why you shouldn't be learning or even using already developed technologies in the HTML 5 spec just because they're "not finished yet."
Now obviously I'm not expecting you to jump straight into HTML5 and CSS3 before learning the fundamentals, but keep an open mind and welcome this spec after you feel comfortable enough with the general structure of HTML and CSS.
One very easy to read resource is '
Dive into HTML5' by Mark Pilgrim. Sadly Mark recently left the internet, taking all of his connections and resources with him. You'll still find a mirror up on google of the free guide, but it's not likely to be updated as the spec progresses, so use it more as an introduction and not a realtime reference guide*.
Getting yourself familiar with the planned features will mean your website isn't at risk of getting left behind when the specifications are finished and everyone else has already been using things you're only just learning about.
Most modern browsers are already supporting a huge range of HTML5 technologies, so you'll be missing out on the power of the canvas, audio and even video elements just to name a few.
There's things you can be doing in your markup that may not take effect just yet for all browsers, but will leave you free to ignore changing anything once other browsers do support it.
Also, the
CSS3 spec also technically falls under HTML5, and a huge majority of the features are already supported by most modern browsers; Web fonts, animations and transformations,
advanced ways to select elements, etc. You'll be missing out on all of those features.
* If you're not afraid of heavy reading, the best source for up-to-date information on the HTML 5 spec is on the
W3C website. W3C is in charge of developing the spec (and most other web related specs), so there really isn't a better resource to use as a reference than that one.
Good luck and have fun on your journey, and remember to always test your markup in all browsers (except maybe IE, unless you like the idea spending hours rewriting all of your code).