There are many references to a maximum limit of 50mA when sourcing power from the two 3.3V power pins on the GPIO (just for clarity, I am NOT referring to the I/O pins). However, I suspect this current limitation applies to the RPi boards which pre-date the B+ (summer 2014) and used the legacy LDO VR. I believe this limitation no longer applies to the higher capacity RPi models which use the buck converters.
In this excellent review of the B+ power supply by AdaFruit prepared back in July 2014 ( https://learn.adafruit.com/introducing-the-raspberry-pi-model-b-plus-plus-differences-vs-model-b/power-supply ), she indicates the 3.3V power rail is supplied by the RT8020 converter chip which is rated at 1A (key point). Recall that the external power to the B+ is a 5V 2A micro-USB adapter which supports the entire board, including the 5V power rail that supplies USB ports, HDMI, etc. Based on the rating of the RT8020 chip, only 1A of this total 2A is potentially available to the 3.3V power rail.
In this case, the 3.3V power pins should be able to supply any remaining current capacity after essential power consumers like the SOC and GPIO I/O channels draw their power needs which are <= 3.3V. Just as an example, let's say the B+ board is running without any connected USB or peripheral devices (that would normally be the 5V consumers) and the board is consuming 600mA. In this case, the 3.3V power pins on the GPIO should be able to supply up to 400mA. This is a far cry from the meagre 50mA that is so often and mistakenly cited!
Please comment if you agree, or not?