Hey, mine, too, and recently.  Assuming our SB-600's are about the
same age, maybe it's a glue defect, where the glue lasts for only a
certain amount of time.  If there are more people with this problem
then maybe Nikon will do a recall and do a fix for free. :)

Did you do anything to cause the button to come off?  I myself don't
recall doing anything.  I just took it out of the camera bag one day
and noticed the button had started to come off.

I was thinking of using rubber cement or maybe even just Elmer's white
glue.  I might write to Nikon technical support to see if they have
any suggestions.

Kevin

I glued mine back on with some kind of super-glue. It has held for a
few months now.

Cheers,
DuncanC