I ride with Rudy Projects (onyx? can never recall) with a slight vent in each corner. I have some fogging issues in the winter XC skiing. If I stop after a long hard pull and my jacket is open to vent some heat they will indeed fog then. Riding? never had a problem.
Admittedly, you can get the same deal with much cheaper glasses (though beware of cheapo lenses, the don't protect the eyes nearly as well). I use the Rudy's because I can clip my prescription lens onto the inside of the frame.