How do you mean,"devalue" the dollar? Technically inflation "devalues" a monetary unit, but also is a natural phenomenon, especially in our type of economy. When you talk about this kind of devaluation, it is less important because, while the real value of a dollar may change, the value of the dollar, outside other circumstances, does not change in relation to the value of other monetary units subject to like inflationary pressures.
Are you talking about declaring national bankruptcy? Are you speaking long term or short term? If national bankruptcy is what you're concerned about we can talk about the actual likelihood (or lack thereof) of this happening.
A little more info, please.