The FTC is taking public comments on a national "Do Not Call" list. A telemarketer who contacted someone on the list would be breaking the law and subject to suit or prosecution.
The page I just referenced is also a great resource for knowing how telemarketing is regulated in general. Your state may already have a "Do Not Call" list, for example, and you can let the three major credit bureaus know you don't want your personal information shared.
If you're interested in shaping this law, make sure your comments are among those made.