http://www.box2d.org/manual.html#_Toc258082974
"8.13 Rope Joint
The rope joint restricts the maximum distance between two points. This can be useful to prevent chains of bodies from stretching, even under high load. See b2RopeJoint.h and RopeJoint.h for details."
I would love to see some Rope joints, i'm sick an tired of making chains that stretches for miles using pivot joints (revolute joints). I think it's time for the rest of Box2D goodness to get ported to corona.