pito:
@Nick: a quick comment - when you measure the voltage drop on a 1Meg resistor pls do consider a standard meter has ~10Meg internal resistance. So, 1Meg || 10Meg = 0.909Meg, thus your nA values will be 9% off..
That's a very good point, and it shows the flaw in my plan. I don't normally measure current like that, and was trying to help the OP resolve whether or not his meter was wildly out. The uCurrent device doesn't have that flaw, as it amplifies the voltage drop across its internal resistor.
In fact, my reading was out by about 10% and you have explained why, thanks!