Gord Goodwin aka feelgoodcomics has released very nice and useful script for weights normalization.
Normalizes the entire mesh (per vertex) by distributing all weights evenly, or preserving 100% groups based on 3 rules:
1.)If there are no 100% groups, all weights are evenly distributed.
2.)If there are more than one 100% groups, they are divided evenly, and all other memberships are removed.
3.)If there is just one 100% group, it becomes the owner of the vert and all other influences are removed.
It also provides a "mode" which allows you to update the selected/active group and normalize its weights by either preserving the weights entirely, or distributing them evenly. Non-bone groups are left untouched.
As well, the peak (100%) value can be set, and there is a tolerance setting to determine the minimum value of a 100% weight.