In reply to Basemetal:
A very good question...
The answer is that it'll fit some, but not others, and even those it does fit it'll be very tight - hence not something you'd want to be doing with cold hands.
Unsurprisingly it fits Petzl helmets such as the Meteor and the Sirocco. I also tried it out on one of Black Diamond's new Capitan helmets, which are coming out next year, and it just fit a size one, but there was no way it was fitting on a size two.
As such, it probably won't be the first choice for climbers, although the irony is that once it is on its low profile nature means you're far less likely to bash it.