Well....seems to me that BAR isn't only testing a all-new rear sectionof the car for 2004....if you pay attention to the pictureyou'll see a red dot on the tarmac....what is it?
http://f1.racing-live.com/img/imgday/10.jpg
It's a laser ride high sensor, first used by Mclaren in 98 or 99 (not to sure when the became a frequently used item.....one of these years was the year when teams started confirming that they used this system).
Because of this pic I suspect that BAR is also testing a new front suspension geometry.