Also, Title 31, sec. 5112, paragraph (k) of the U.S. code explicitly authorizes the Treasury department to issue as much money in the form of platinum coins as it wishes, at its discretion.  So, in a pinch, Geithner could always just stamp out a few platinum coins marked "ONE TRILLION DOLLARS" each, and make the debt problem go away.

(Source: http://www.law.cornell.edu/uscode/31/5112.html; some discussion at http://www.alternet.org/news/151718/there%27s_a_solution_to_the_debt_fight_that_could_avert_catastrophe_--_why_is_everyone_ignoring_it/)