With the onset of cheap 16-bit, 18-bit or even 24-bit sigma delta ADCs, it isn't a very hard circuit to shove a 1 Ohm sense resistor, 2.048V reference and have sensing down to 31uA (16-bit), 8uA (18-bit) or less for 24-bit ADC.

SigmaDelta gets pricy if you want higher speeds though. But it's possible.