One thing to keep in mind is that many institutional investors, including those in Europe, are required to invest exclusively into triple-A instruments. This downgrade means a major sell-off of US bonds and whatnots currently held by such investors, and that could have an interesting avalanche effect.
That's not going to happen.
Plus S&P's logic is shaky on this. The whole reason the threat of S&P dropping our rating has had no impact is because their demands were impossible to achieve. Cut $4 trillion from the budget in 10 years when we're expected to add $9 trillion in the next 4? Not possible and everyone knows it.