From the TSA website:
Passengers who do not or cannot present an acceptable ID will have to provide information to the Transportation Security Officer performing Travel Document Checking duties in order to verify their identity. Passengers who are cleared through this process may be subject to additional screening. Passengers whose identity cannot be verified by TSA may not be allowed to enter the screening checkpoint or onto an airplane.
About 4 years ago I was at the airport without any ID and simply got SSSS'd. It actually was faster than going through the normal line. However, I do not know, nor have I heard what their policy is today. The website states that they MAY deny boarding. - not very helpful.