Assess your target audience. Are they likely to be early-adopters? Is it critical that all visitors to your site are catered for? Or will a few not mind being met with an inconvenient, yet polite "please upgrade your browser" message?
For business sites I'd say no. Not yet, anyway. You probably can't afford to lose users of older browsers.
For a personal website or project, why not? It'll be great experience learning the latest HTML5 features, and you'll be ahead of the curve when it does become mainstream. Besides, more people writing sites conforming to HTML5 means more pressure on users of older browsers to upgrade, benefitting all of us in the long run.
