I've just ordered a bunch to see if I can get better resolution using analogRead().
They're talking about a different inductor (a ferrite bead on the USB power rail. Uno and later Arduinos have this, I think.)
How sure are you about the pinout of your USB connector? I notice that the schematic symbol you're using doesn't match the one from the official schematics, nor does it match the physical pinout (power is on the "outside" pins), and there have been issues (back in the diecimilla days) where the actual pinouts of the connectors were "confused."