A Boston law firm, Beck Reed Riden, has developed a chart that outlines the law of non-competes in all 50 states. It is pretty good tool and an easy way to get some basic information about non-compete law. Review of information related to states with which I am familiar suggested this chart is accurate. This is just a starting point for information but every search begins somewhere.
UPDATED (9/5/2011): The chart was updated as has this blog post.