This article is a must read for anyone who freelancers and/or sells services through Paypal. While she makes note of her experience with Paypal, it's good to keep in mind that this experience can be had with anyone accepting credit cards online for non-tangible items.

Know your rights and what you're getting into: http://freelanceswitch.com/the-business-of-freelancing/freelancers-beware-of-receiving-payments-via-credit-card-through-paypal