Yep, like they said, it really is that narrow but also very fun to ride.
The gopro website appears to have been hacked but I found thes images from google from a couple of helmets
You stick a plastic base plate on the helmet which uses 3M adhesive and is curved to fit the helmet and then you just clip the camera mount into it.
You get two curved helmet base plates with the motorsports kit and 3 flat ones for use on the bike, with other kits you get different bits so it's really personal choice about how you see yourself using it.
I don't mount mine on my helmet as i prefer to mount it on the bike.
I bought the motorsports kit and then bought a seat post and handlebar mount seperately so that it could be mounted on the SDs handlebars.
I bolt a piece of perspex with one of the base plates fitted on it in place of my numberplate hanger for track days which works brilliantly for rear facing shots and means it can be removed and reused easily. It was Superdans idea though so I can't take the credit for it though.
I will probably get a vented helmet strap too for snowboarding and will use the handlebar clamp on my unicycles too.
Also interested in the roll bar mount as it's the right size to clamp round the fork leg to get some different angles.
I got mine cheap from an american seller on ebay who sent it as a gift but there is a guy on KTM forum who is selling the motorsports kit for £255 which is the cheapest you can get em in the uk.