A few months ago Mothering Magazine had an excellent article on breastfeeding in public and the law, written by Jake Marcus. The cover of that edition is to the left, with Emily Gillette on the cover. Mothering has put that article up on their website, so you can check it out even if you're not a subscriber.
The topic of nursing in public and the law is one that confuses a lot of people. I thought that Jake explained very clearly why laws protecting the right to breastfeed in public are needed:
There are no laws in the US forbidding breastfeeding outside of the home, and only two states in which laws place any limitation on the way in which public breastfeeding may be done. However, in the absence of a law establishing and protecting the right, a woman who breastfeeds in a public accommodation—a privately owned place open to the public, such as a restaurant or shopping mall—might lawfully be asked to leave, either by the owner or in accordance with the owner's instructions...
A basic maxim of American law is that a right without a remedy is no right at all. In plain terms, this means that although you may have a "right" to do anything not otherwise forbidden by law, if you do not also have a legal protection against someone interfering with that right, your ability to exercise it may be limited.
Mothering is also making available a cut-and-fold pocket guide to breastfeeding in public in the U.S., including notes on the TSA policies regarding bringing breastmilk on board airplanes.