This post from flurry [1] suggests that a big chunk of revenue is going to a small proportion of very successful developers (there was a HN post about which companies they were but I can't find it now).

That said it also suggests that revenue is increasingly moving down the long tail which may be a sign small app companies may become more viable.

[1] http://blog.flurry.com/bid/88014/The-Great-Distribution-of-W...