I can remember exactly which attribute it is in the soft bind tool, but it is a threshold that indicates how far away different joints affect a given part of the mesh. In this case, what is happening is that the mesh on the fingers have some weight attached to the joints in the next finger. I will be back at my main machine later tonight and will provide more details.

You could also try painting weights to make sure that the joints in one finger are only affecting the mesh in that finger.

Learned all this from Mike's rigging tutorial

Dan