I have a rush hour and have had the exact same problem. It's happened a couple of times during skidding, but the slippage is only about 1/8 of a full crank turn (hope that makes sense). Then when I pedal hard it tightens back up. I've taken it to the lbs and got it tightened and then it's good for a while. The cog and lockring were quite loose when I got it, so I'm not sure if the hub is stripped or just loosens with time.

The lbs guy swears it's not stripped but I don't think he deals with fixed much. If you feel it slip again just don't skid or back pedal and then tighten it until you're ready to upgrade the wheelset.