Adam Ozimek says that despite what you may have heard, Fitch has not affirmed its AAA; they're still reviewing the matter. (Moodys is also apparently threatening to downgrade us by 2013 if matters don't improve. Ozimek identifies what I think is the key question:
So despite media reports like those above, Fitch has not "formally reaffirmed" the AAA rating, and a downgrade appears to be on the table for their upcoming review.
One thing to consider is that an S&P downgrade could make a Fitch downgrade more or less likely.
But which is it? I can see it two ways:
The S&P downgrade radically raises the consequences of a Fitch downgrade. As I understand it, contracts and regulators that require AAA tend to require it from two out of three of the ratings agencies. That makes Fitch the swing vote. They might hesitate to be the people who trigger a financial crisis.
On the other hand, there's the herd factor. If Fitch ends up downgrading the US later, they look stupid for not seeing it sooner. They might want to get ahead of the curve.
I sure hope they're more worried about markets than their retrospective reputation. But I do not generally expect to find altruism in financial markets.