Hmmm the Pinlock system seems reasonable:
http://pinlockusa.net/Home
Seems like you buy one of their fog shields that accept inserts that cut out the fog. Pretty interesting. For all you cheapos out there I have also found the unknown answer to your fogging problems. I think that Rain-X antifog stuff if absolute junk. It's a pain in the butt to put on and it stinks. I don't even know if it works in helmets but I tried it in my truck once and it was a mistake. So what do I use if I have a shield that isn't anti-fog from the factory? Well the best thing that I have found that prevents fogging is SHAVING CREAM! Yeap its' true. Grab any can of shaving cream and wipe some on the inside of your helmet lense. Everyone has the stuff so it won't cost you a dime.... AND IT WORKS!!