I know the reaction will start elswhere, specifically east of the International Date Line, but it will work it's way around to New York and that is where the action will be.
First the big caveat. The word 'unprecedented' means no one 'knows' what will happen, it is all guesses and opinion, informed and otherwise.

In a nutshell we are looking for:
Treasuries okey-dokey.
Equities get sold.

If you haven't been following the Live-blog at MarketBeat, hie thee over.
MB started yesterday morning for the Jobs report and after the S&P rumors started racing up and down the street stuck with the format.

In addition they are doing individual posts such as this, bylined Mark Gongloff, at 10:35pm last night:
S&P Downgrades the U.S.: Five Things
Here he is at 1:38 this afternoon:
Downgrade Q&A: Is AA+ So Bad?