Exotools creators of Exoswitch, a new bidirectional contraint plugin for Autodesk Maya recently previewed the plugin at Siggraph 2010. The Exoswitch plug-in introduces a new constraining paradigm with true multi-directional capability that is unlike current constraint systems, Animators and Riggers are no longer bound to the limitation of the single-direction, one-way street relationship between driver and driven transforms. The ExoSwitch Constraint has at its foundation the ability to dynamically create bi-directional relationships between driver and driven nodes and animate those relationships. Check out the features for Exoswitch at the Exotools page here, as well as here for the FAQ that answers things like where, when and for what.
Michiel Duvekot
I saw these guys at Siggraph and they were kind enough to give me a demo. This will change the way we animate and make it so much more natural and intuitive. Very nicely integrated in Maya. Clean, elegant, beautiful.