F1 2009 Video - Technical Details
Note the part about KERS (Kinetic Energy Recovery System). If the front brakes do the majority of the work it would at first blush seem that KERS would be more efficient on the front of the car.
On the other hand, with a rearward weight bias and center of (aero) pressure, combined with minimal weight transfer under braking compared with the higher and softer suspensions of other cars, not to mention the fact there's no space up front, and it was inevitable the packaging constraints would require regeneration to take place via the rear wheels.
Anyway, have a look at the video: