There are too many salary disclosure laws in the US to fully understand. Compliance is difficult, and the penalties for non-compliance are severe.
Here is a great link that is helpful for those who want to work within the confines of the law. Littler provides a broad array of employment-related law advice, and you can sign up to receive email updates as well. This is also a great resource for your smaller clients who may not have in-house HR professionals or ready access to current legal information.