My two SGs have this exact same problem (one gibson, one epiphone), almost in the exact same place; my dead spot is on F, most prominent at the 10th fret of the G string. Several SGs I've tried at various stores and such have had the same issue, too. It drove me crazy at first but eventually I learned to play around it (the guitars are too nice to neglect just because of one dead note). It seems to come and go with humidity and weather changes, too. I also managed to eliminate it by using a "Fat Finger", basically, a C Clamp which you attach to your headstock and which adds mass to the guitar. The fat finger sits in the closet, though, I've gotten over the dead spot more or less now.
