This issue is very complex. In the Ideal a lower $US would lead to increased Exports, but the current situation is far from Ideal.:
1) As Supertool and Kibbo mentioned/explained, the biggest Trade deficit is with China and no matter how low the $US goes, the current Yuan relationship to the $US prevents any kind of advantage for the $US. 
2) When you go to Walmart(or other large chain) to buy things, you'll notice very few "Made in USA" items. There will be some, no doubt, but the amount of "Made in.." China, India, or other place items will be quite telling of a few things:
a) Much of the former US Consumer Production has been Outsourced/Moved
b) There are a wide variety of Product segments that simply have no US based competitior at all. Meaning that no matter how low the $US goes, certain Products will always be Imported.
3) One major issue with a low $US is that while Oil priced in $US remains relatively stable, at least based upon Currency value(Supply and Demand are different considerations regarding Price), the Price of Oil is actually going down for European and Asian competitors(except for China), as they now can buy a lot more $US for Oil(and other Commodities) Purchases.
4) As already mentioned, as the $US slides, the Purchasing Power of Americans decreases as well. OTOH, the Purchasing Power of Europeans and Others increases. This is a Major issue for the US since its' Economy is very much focussed on Consumption. If Consumption is reduced, so is Economic Activity.
5) It would take years just for the US to begin to take advantage of the Competitive advantage of a lowered currency. In the meantime Inflation could become rampant and take years just to fix causing even more problems. 
6) If the $US is abandoned as the International Currency for Trade in Oil or other Commodities, there would be a loss of Cost Stability in Commodities that the US currently enjoys. Producers of Commodities priced in $US are tempted to do just that, since many actually lose some of their Profits due to the weakening $US. 
7) Much of the US success has been in attracting Experts in various Professional fields from Foreign sources(Canada, Europe, Asia) because the stron $US made Economic sense to these Experts. With the devaluation of the $US, these same Eperts may find that going back to their Nation of Origin now makes more Economic sense, thus causing a Brain-Drain from the US instead of the reverse.
I can see the value of a lower $US in relation to the Trade Deficit, as it is totally out of hand, but while the Yuan is valued as it is and with equally large Government Deficits requiring large Foreign Capital inflows the Short  to Midterm Economic prospects of the US looks rather grim. Add in the High Cost of the Iraq War and other looming Costs the situation just gets grimmer(more grim?).