I just learned that personal auto insurance policies do not typically cover accidents if you are driving a vehicle with a magnet or sign.  A commerical auto policy is required.

Check out the business insurance section of Florida West Insurance's website, or their newsletter for more info:

http://www.floridawestinsurance.com/business.asp
http://www.floridawestinsurance.com/images/mar2008.pdf

If you have car magnets displaying the name of your business, you'll definitely want to talk with your insurance agent to make sure you're covered.