GregF, I just happened to hear on CSPAN today that many of these statistics can be obtained from the CIA Factbook. here.

For the United States it says: Protestant 52%, Roman Catholic 24%, Mormon 2%, Jewish 1%, Muslim 1%, other 10%, none 10% (2002 est.)

On this site it says:
Religion: This entry is an ordered listing of religions by adherents starting with the largest group and sometimes includes the percent of total population.

But I don't see the answer to your question, how is religion defined, or what questions were used in determining the facts quoted. There is a statement on the contact page that they will respond to email questions that are not answered in the FAQS. Might be worth a try if you really want to know.