Mine do this and even thicker every night here in Atlanta where the nights are getting hotter and hotter. I am quite a newbee but I think this is two things:
1. They come out of the hive to allow the hive to cool off in the heat - less bodies inside, less heat
2. The new brood that hasn't started foraging yet hangs out on the front porch like teenagers, bopping and washboarding and sometimes fanning the hive to build up their strength before they begin their foraging behavior - so it's a sign that you have new babies from the inside of the hive, now moved out.
Michael B educated me that this is not a beard - at least not yet. He looked at this http://photos1.blogger.com/blogger/6269/2875/640/DSC00866.jpg
and said that this was nothing to worry about.
If you have SBB, adequate ventilation for the hive, and room to grow (empty super) then they are doing 1 or 2 above and not trying to leave.
Linda T learning more and more every day about bees