An extremely oversimplified answer is that it would hurt confidence in the dollar, which would further damage its status as the foreign federal reserve currency, which would cause further inflation in the US.
When foreign federal reserves switch their reserve stash from dollars to something else, the dollars are basically "created" on the market as one currency becomes instantly less available and the other instantly more available.
http://www.economist.com/finance/displaystory.cfm?story_id=1...
An extremely oversimplified answer is that it would hurt confidence in the dollar, which would further damage its status as the foreign federal reserve currency, which would cause further inflation in the US.