Should US government debt be rated junk?

This article makes the argument that US backed debt is not as strong and reliable as it once was. The author even goes as far as to compare US and Greek debt and deficit structures. Is this really a fair comparison? Was or is the U.S. economy likely to default on any debt they have ever issued? I agree that the deficit the U.S. government is running is excessive. I also believe that the U.S. should borrow the EU's ideas for reform and cut public spending and raise the age people start to receive social security. I do not believe the US debt is ever in any risk of be defaulted on.