LadyAda made a guess that the regulator is an RT8020A: https://learn.adafruit.com/introducing- ... wer-supply
That regulator has a low dropout mode, where the voltage drop is determined just by the resistance of the regulator P-MOSFET (0.28 Ohms). My model-B draws 400mA under load with no peripherals. Assuming the B+ draws the same current (when the input voltage is so low that the regulator is no longer switching), the dropout should be about 100mV.
In other words, a B+ ought
to run with just 3.4V on the "5V" power input. (To be tested when my B+ arrives)