I was under the impression that the light sensors were in the speaker grilles at the side of the keyboard. Not sure why, I think someone may have gestured towards that area in one of the videos. I may be wrong tho.
Either way, I could use a Pro Keyboard with glowy letters. That would be v useful. Perhaps not as a standard if its costly, and unneccesary in well-lit offices, but certainly as an option for home/cave users.