The long-term implications of a US downgrade

"David Boucher, of The Economic Word, asks a very good question via email, about the implications of the US losing its triple-A rating:

I was wondering if the state-level impact of a debt downgrade would be different, more severe.
From my understanding, Illinois and California have the two lowest ratings of the US states; if the US were to be downgraded and so would a couple of trouble states, would it have an impact on the ability of the Federal Government to lend a hand to those in trouble? And as a result create a Euro-ish type of debt crisis within the US?

There’s certainly a general understanding, in the markets, that California is too big to fail: if push came to shove, the federal government would bail it out rather than let it default. But David raises a good point: is the moral-hazard trade going to get weakened if the US loses its inviolability?

The way that credit ratings work, any municipalities which currently have triple-A ratings would almost certainly lose those ratings were the US sovereign to be downgraded. As far as I know, there’s no precedent for a sub-sovereign entity to have a higher rating than the sovereign, except in extreme cases where the sovereign is actually in default..."

at http://blogs.reuters.com/felix-salmon/2011/07/28/the-long-term-implications-of-a-us-downgrade/