The US government cannot borrow forever. Eventually, the cost of borrowing will rise and the budget will collapse.
But why would the cost rise? The US has an outstanding record on repaying it's debt so lending money to the US government is a very good place to put your money for people who don't need all their money to be easily accessable. Unless a Ron Paul type gets the presidency the lending could go on for some time.
I get the impression that once Europe stabilizes it's crisis, or the euro collapses, the world's gonna take a hard look at America and it's lack of long terms solutions to it's debt.