man, i started writing a response at 3:00.... and this went on for 2 hours, while i was oblivious to the new posts, here's my take...
Correct me if I am wrong, but isn't the value of the dollar these days based on trust. The american greenback has been, and for a long time at that, the standard. When back then, it was gold, and silver. So therefore, the dollar is 'backed' by the strenght of the 'trust' other countries have in your dollar, or the hope that the american economy is strong. As other countries have been losing interest in the greenback(due to MANY factors I'd love to jump into, but won't), the world's trust, and consequently the value of it, has taken a hit.
4 Influences on the value of the dollar: (IMHO)
Debt (loss of trust, would you invest your $ in something that's always in the red)
War (it worked in the first and second world wars, but that strategy doesn't work anymore, ask ****[starts with B and ends with ush)
Economy (well, how the economy does is actually based on the value of the dollar, not the inverse)
Import/Export (more money coming in from abroad (export), and the less going out (import) )