For some reason, I forgot about team 522, the Robowizards. They used tracks from 2003 to 2008 and they had a very effective system. (World Champs In 2006, IIRC)
Their 2007 iteration was quite clever and it solved the turning issue with the addition of a Ball Caster That was lowered to the ground with a piston and this allowed the drive train to lift itself up onto just the rear most portion of the tread allowing for efficient turning.
Here's a link to their previous robots in the CD Image gallery.
http://www.chiefdelphi.com/media/photos/tags/frc522