Preventing intersection in Auto-IK

Hello there. I’m trying to find my way in blender 2.5x and I’m currently puzzled over this:

Let’s say I have four cubes in a row, each one assigned to different (but connected to each other) bones. The effect of Auto-IK is to have this series of cubes behave like an arm, or a chain, or a worm, or something similar. My problem is that I don’t want these cubes to intersect. I want them to be constrained (magic word?) up to the point that when one cube touches the other, they stop rotating/moving past that point. I remember in 2.4x there was a way to automatically do this (but I also might remember wrong - it has happened to another one of my posts :P).

Any ideas?