http://blenderartists.org/forum/showpost.php?p=1382653&postcount=102
(todo: expose this setting in World setting GUI)
Expose contact processing threshold in Advanced GUI, next to rigid body margin, called CPT.
Default to 1, makes rigid body stacking a bit more stable, smaller values makes sliding easier (at the cost of easier jittering).
Disabled for 'dynamic' objects that don't rotate, because characters etc. always need smooth sliding.