The President has always had the authority to place the country under martial law, but under incredibly strict conditions. If conditions ever become so bad that martial law is declared, most people will be glad to see it, not disappointed.
Read up on the Insurrection Act of 1807 to limit the President's power of martial law, as well as the Posse Comitatus Act to prevent the government from using military force against its citizens. And lastly, consider the significance of the 2nd Amendment in the face of this rumor.
As for rumors, there's still rumors about flying saucers and chupacabra. You'd be hearing it on network news and there'd be a stink all over Congress, especially since there'd be no justification. Blogs are entertaining, but they're still opinion. Don't let them feed your fears.