I think it's a good-faith concern. As much as I believe in data, the ability of the US to borrow cheaply enough to finance deficit spending is entirely up to the whims of investors. People are fickle. If the shit hits the fan and investors lose confidence in the USA for whatever reason, the deficit spending we currently rely on could become many times more expensive, and unsustainable.
EDIT: Downvotes? I doubt most people are even aware of this 90% number, but everyone knows about Greece. If you ask an average tea partier, I am willing to bet they are more concerned about Greece than exceeding 90% debt/GDP. If this is nonsense, refute it (I'd like to hear, and believe, that refutation).