The debt and the 14th Amendment

[Anonymous]

Section 4 of the 14th Amendment includes this sentence:
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.
There is an argument (by Senator Schumer, for example http://www.huffingtonpost.com/2011/07/01/chuck-schumer-14th-amendment_n_888692.html that this means we cannot default, so there is a constitutional requirement to raise the debt limit. Do our resident constitutional scholars think this argument would fly, and whether it might be a valid plan b if the debt limit negotiations fail?