It doesn't look like there are. I searched Newegg for 15.4" widescreen laptops with dedicated graphics and a Core 2 Duo and the lightest one that I saw was 5.5lb.
I think that the MBPs advantage comes from the LED backlight on the screen. It is apparently lighter than the traditional CCFL Backlights. Also, the aluminum shell is, from what I have heard, actually lighter than most plastic shells. That is why the MBP is not significantly heavier than the MB even though it has a whole lot more under the hood, and a bigger screen.