Nice gem of an interview with an employment lawyer from the always smart Marci Alboher , author of the New York Times blog Shifting Careers:

Can Ex-Boss Object When Ex-Employees Start Businesses?

This is often a very sticky issue, and I suggest you really follow the advice described in the article.  Nothing would be more of a bummer than getting mired in legal trouble with your ex-employer while trying to launch your business!

And the legal issues are not the only thing to keep in mind:  it is not a very good strategy to leave a former employer on really bad terms.  Unless they have been absolute maniacs, do your best to leave on cordial terms.  I started my consulting business by being hired by a former boss, and many of my old employee colleagues were instrumental in spreading the word about my new business.

Even if they take the low road, you take the high road.  Feels better, and I think it is better for business.