There are new attributes for blendshapes in Maya 2012:
A new set of component weights have been added. These weights multiply the various target weights, allowing the user to quickly paint out regions where no target has an effect.
The base weight is an overall multiplier for the targets.
Therefore a zero weight on the base would mean no targets (regardless of their weights) have any effect.
A base weight of 1.0 (the default) would act normally, a weight of 2.0 multiply them by two, -1..., etc.
The blendShapes targets can now be assigned a 'normalization group'. Targets in the same group have the sum of their weights normalized to 1.0 when editing via the Component Editor or the Paint Blend Shape Weights tool). Optionally each group may also use an extra set of weights to include in the normalization calculation. These weights are associated with the group itself, not any particular target.
The normalization weights appears in a new columns in the Component Editor and as new paintable targets in the Paint Blend Shape Weights tool once it is created via the Edit Deformer -> Blendshape -> Edit Normalization group .