I'm not sure its true but I heard on the radio that McCain would also be violating the law if he received an unsecured loan. A law like that would make sense because without it, a donor could offer an unsecured loan, get around donation limits that way, and never expect the money to be actually paid back.

 McCain says he's not obliged to public financing limits because the loan he received was not secured by public financing.  Either way, McCain is breaking the law.