bug in 2.58

hi, i found a bug in this new version of blender.
i try in different models: i assign two vertex group(left and right) and this works well. when i create different shape keys to the model(for example smile shape) and try to assign the shape to left and right group, this not works. in 2.57 version it works well.
is it a bug? how can i fix it?

thanks a lot