I use my ski helmet as my open face...I started that as a student, because I have realy big head and could not get ANY of the protects they had on! I use a beori full cover racer, rated to 60mph+...actualy offers better protection then most skydiving helmets, and is realyrealy comfortable..only 2 problems with it, no place to attach my pro-track, and it can get kind hot in the summer.
I would VERY STRONGLY caution against using the beanie style half helmets favored by most snowboarders these days. the way they are designed there is a large (1 to 2 inch) lip all the way around the head, which could easily get caught on opening... "You can blow your nose, and you can blow your friends, but if your wife buys too many pairs of shoes, your head could explode."