Mercedes has this now for some panorama sunroofs (MAGIC SKY CONTROL)!
http://www.mbusa.com/mercedes/vehicl...nology#modules
"A panorama sunroof with MAGIC SKY CONTROL alters its transparency by uniformly orienting particles embedded within its glass panel when electrical current is applied. At its darkest setting, the thermal energy entering the cabin is less than a household light bulb. And even in its fully transparent mode, both UV and infrared rays are effectively blocked, helping keep the cabin cooler."