The folks at The Boy Genius Report spent some time poking around server logs and found references to iPhone OS 2.0.1. It seems that build 5B101 is the 2.0.1 software for first-generation iPhones, while 5B103 is for 3G iPhones.
No word on what's included, of course, or when this might hit iTunes. According to AppleInsider, some people are having trouble with the camera and the accelerometer since the update. Have any of you 1st generation iPhone owners had software trouble since the update? Here's hoping this update will solve the problem.