Since our currency isn't fixed (meaning the value floats), Timmy doesn't get to announce a new value, higher or lower.
Re: our currency floats
some of the things that help it “float” one direction or another:
US backing for world currency stuns markets
http://www.telegraph.co.uk/finance/economics/5050407/US-backing-for-world-currency-stuns-markets.html
US Treasury chief Timothy Geithner says America will not engage in dollar devaluation
http://www.telegraph.co.uk/finance/currency/8072427/US-Treasury-chief-Timothy-Geithner-says-America-will-not-engage-in-dollar-devaluation.html