Set default constraint solver mode more compatible to Blender 2.48 settings, this fixes rigid body stacking in this blend file:

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.

source/blender/blenkernel/intern/object.c

@@ -984,6 +984,8 @@ Object *add_only_object(int type, char *name)
 	ob->anisotropicFriction[2] = 1.0f;
 	ob->gameflag= OB_PROP|OB_COLLISION;
 	ob->margin = 0.0;
+	/* ob->pad3 == Contact Processing Threshold */
+	ob->pad3 = 1.;
 	
 	/* NT fluid sim defaults */
 	ob->fluidsimFlag = 0;