Full faith and credit, or worthless IOUs?

Josh Marshall points out:

There was so much bamboozling going on tonight in that press conference that it was easy to miss one essential contradiction in the president's argument. You don't have to worry about private accounts, he said, because if you want you can fill your account with US Treasury bonds which have no risk at all. They're backed by the full faith and credit of the US government. But he says that the very same Treasury notes, when they're in the Trust Fund, are just worthless IOUs.