This IMF factsheet is dated September 13, 2011 and states,
"The NAB is a set of credit arrangements between the IMF and a group of member countries and institutions, including a number of emerging market countries. The NAB is the facility of first and principal recourse in circumstances in which the IMF needs to supplement its quota resources. Once activated, it can provide supplementary resources of up to SDR 367.5 billion (about $591 billion) to the IMF.
The expanded NAB came into effect on March 11, 2011, and was activated shortly after for a period of six months, in the amount of SDR 211 billion (about $339 billion). The General Arrangements to Borrow (GAB) remains in force and can be used in limited cases."