if the US $ drops in value, it's actually something that may not be troubling to the US administration - as it would affect the trade imbalance to the US. For instance 85% Canadian exports are to the US
- a rise in value of the Canadian $ hurts the Canadian economy more, even though some imports may be bargains.
The other benefit to the US is the value of debt is lower so
when paying off debts (say to the Asian Countries) with a lower US$
its less to pay. (And I believe that it has dropped 20% since GWBush
was appointed.)

China isnt affected as much, as their dollar is pegged to the US $
(and when you consider the avg. wage in China is $55 US per month
North America and Europe cant come close to competing)

The US basically enjoys the advantage of a currency used around the world. But if the dollar drops really low, investors may pull more of their money out, and switch to another currency.