Got one of these last week and thought I'd mention it in case its useful to someone else.
http://www.webbikeworld.com/r4/sunax-sunshield/
http://www.sunax.de/en/
Its a tinted polycarbonate strip that fits between the helmet and the liner at the top of the face opening.
Alternative to sunnies, which I don't like under a helmet, and a tinted visor which is $90 for my helmet.
Works well. Only real drawback is in a tuck I have to look through the tinted bit.
Spoke to the Aus distributor and he says he should have them in shops soon and he'll be at PI.
(He had an ad in 2 wheels)
I know there are alternatives but this is pretty neat.