Well, that's super odd. The B0 pin is connected to an LED (in a pushbutton which has a built-in current limiting resistor—
adafruit.com/product/559). When I disconnect the LED and measure the voltage on the pin, I get 4.7V (with a supply voltage of 5.0V, odd thing #1). Connecting the LED just by itself to another 5V supply it lights up fine drawing 13mA @ 4.8V.