There is nothing saying that the US Dollar will stop falling ...
The US Federal Reserve (foolishly) has a dual mandate to protect the value of the dollar and to prevent economic downturns. The problem is that inflation is more publicly acceptable than a recession and the Fed has been running an inflationary monatary policy since the mid 90's to prevent recession. Now, there are two reason's people haven't felt the impact of inflation in the US; countries like China, Japan and Saudi Arabia have been proping up the dollar in order to increase their exports and in recent years the housing bubble allowed Americans to increase their rate of spending faster than their income rises.
With the housing bubble expolding the fed is now (very) worried that this will kill off consumer spending, and lead to an unpopular recession; this will lead to the fed lowering interest rates for the next several months. The result of this will be that the American dollar will drop much further and commodity prices will rise in American dollars to record prices; this will cause a new crisis and the fed will be worried that a $110 to $125 Barrel of oil will kill off consumer spending, and lead to an unpopular recession.
The US has been running away from a recession since the mid 90s and it won't work like it used to. Every step away from a recession at this point in time will be felt in the value of the dollar; eventually, if the dollar falls far enough (fast enough) centeral banks around the world will dramatically lower their holdings of American dollars which would be an unrecoverable shock to the value of the dollar.
