Currently there is only one federal province (Lower Austria) where a law has been passed, which makes helmets compulsory. This law will be enforced by the 1st of March. For all other parts of Austria helmets are only recommended, (even though there have been lots of discussions).
That law only applies for youngsters until the age of 15.
Of course, this may change but for now it appears that helmets will only be mandatory in Lower Austria - the region surrounding Vienna in the East of the country - which affects relatively few ski resorts (not including Niederau or Mayrhofen).
If there are any further developments, I'll append them to this post.
[amended with the applicability to youngsters - Admin]