I developed the same problem today! Thought it was the alternator (found the rear race had failed) and relaced it. Then I removed the alternator wire harness and ohm'd it out and it looked good (has a 12G fuseable link). The battery light is still on.

Battery is fine..
Tomorrow I'll check the voltages at the alternator and battery. If the battery voltage (with the truck running) does not look good at the battery I'm considering running a jumper from the B+ alternator terminal to the start relay, then to the battery checking to see if the light goes out..
Any suggestions would be appreciated..
Thanks a bunch!