The May 28 Daily Treasury Statement shows that the US Treasury has an operating cash balance of ~$16 billion which is roughly one day's average cash outlay of about $17 billion. That same statement showed that we are $25 Million shy of the new debt ceiling.

Since the onset of Expanded QE3, the Fed has monetized 73% of Net Public Treasury Debt Issuance, should hit 100% by Mid-July if it continues at this pace, as the Fed has indicated it will continue into the summer.

The US government can't control foreign holders of US IOUs (US Treasury Bonds) who have a growing perception they will never be paid back on the trillions of dollars of debt owed them. Foreign holders of US Treasury bills can sell them off at a loss to others.
 
The amount those T-bonds are discounted is the amount the US dollar will be valued at in the world marketplace. At that point, the US faces a default on its obligations to pay back lenders.