Probably not as big of a deal as it seems. Most other credit ratings agencies had already downgraded US debt, so it's not exactly providing new information to the market.

What I recall from previous downgrades was concern over certain large funds (like pensions) only being allowed to hold AAA rated bonds. They would then have to dump rather large treasury holdings.

Two things can happen to make that relatively unimportant: 1) the funds can change their rules and 2) other large buyers can step in at a very similar price point because the Treasury market is unfathomably deep.