According to a November 2007 report by a senior official at the FDIC ...
"We don't know exactly what portion of the U.S. population is unbanked and underbanked, but we do know that it is substantial. One recent study estimated that there are 28 million unbanked people in the U.S. and another 45 million underserved who lack adequate access to credit. The Federal Reserve has estimated that up to 10 percent of American families are unbanked. We also know that in the U.S., a disproportionate share of the unbanked and underbanked are minorities. Studies have found that 46 percent of African Americans and 34 percent of Hispanic Americans do not have an account at an insured bank or thrift."
The Federal Deposit Insurance Corporation (FDIC) website is found here.
The American Bankers Association (ABA) offers this page for banking consumers that provides basic banking information, security, helpful tips, and other resources.
Additionally, the Federal Reserve System offers this page regarding banking information and regulation. Much more information can be found on banking at the Federal Reserve Bank of Boston site on banking.