The XBR3 and XBR2 (current models) are the exact same television.
The XBR3 has a piano black finish, a black remote, and a 18 month (not 24 month) warranty. The XBR2 has a standard silver bezel that is interchangable, a standard silver remote, and has a 12 month warranty.
The only "improved contrast" could be the fact that the bezel is indeed black, which often tricks the eye into believe the image is more "contrasty" than it truely is.
It's all cosmetics.