http://www.html5test.com/ measures how well a browser implements the HTML5 features.
My copy of Safari 5.0.1 gets a 208 and the Webkit nightly get a 242.
Most of the non-support is in local devices and microdata. I wonder how well IE9 beta scores?
I've started using HTML5 features in a new website I am working on; the only drawback is, as always, older IE versions for which support and especially CSS3 support is non-existent. If only the IE world to switch to another browser or at least use http://www.google.com/chromeframe