I know that Jonathan Turley has a lot of fans, so here are his thoughts:
http://jonathanturley.org/2011/06/29/democrats-raise-constitutional-argument-in-favor-of-raising-debt-limit/
“The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any State shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.”
The argument goes that, by not lifting the debt limit, Congress is “questioning” “the validity of the public debt of the United States.” Under this logic, advocates are encouraging President Obama to simply pay the debts in accordance with the Constitution. That would be an extreme step that would add a constitutional crisis to an economic crisis.
The “authorized by law” clause could present an interesting debate since the debt ceiling is part of a federal statute — though conversely so is the obligation to pay things like social security.
The language is certainly written in absolute terms but it is not likely that a court would rule that it makes a failure to lift the debt ceiling unconstitutional. Congress can argue that it fully intends to pay its debts, but that there is a political dispute over how and when. They can argue that they were not challenging the “validity” of the debt but the priority in the payment. The United States will still be fully liable for the debt and the interest.