As I've repeatedly noted, U.S. credit could well be downgraded even if a debt ceiling deal is reached. See this, this, this and this.
As Zeke Miller and Yves Smith note, the Chicago Mercantile Exchange has announced that it will give less credit for U.S. treasury bills and foreign sovereign debt posted as collateral.
This could be the first tangible effect of weakening U.S. credit.
Note: Technically, the CME downgraded treasury bills based upon concerns about volatility, not their inherent worth.