Was it the HBO documentary 'Too Hot to Handle' ??
They did a ~2min segment on a group of owners (some from here) who used a then-new technique to run a Prius until its tank ran dry.
http://www.greenhybrid.com/discuss/h...light=marathon
Was more of a one-car rally, but they did 'race' agianst each other to see who got the best mpg. It was a very deliberate attempt; not at all typical of what even the best drivers get normally.
The 'pulse and glide' technique they pioneered and proved out with this test is now used by many drivers. Look it up on various threads here to learn how to do it in the Prius.
As for plug-ins, there have been a few news segmewnts on them, mostly about CalCars.