Have you tried staring at a completely black background on your G2400WD in a pitch black room at brightness 100 and contrast 81? I doubt it, almost all TN panels have at least some visible backlight bleed under such conditions, put on a light and the backlight bleed would be hardly noticable anymore, at brightness 50 setting it compares to like my parent's Panasonic G10 plasma TV in black levels (ok lowering the brightness setting on it would make it perfect black but too dark overall), displays have a hard time to really produce perfect blacks in a pitch black room, even on my CRT I had to lower the brightness compared to what I thought was good for use under normal lightning conditions. I did also notice some "purple" blacks at bottom on some of the BenQ G2420HD that my school have recently bought.
Yes, I keep brightness at 70 in daily use and even at 100 there is virtually no backlight bleed that I can detect in a dark room and with a completely black screen image. Since it's a TN panel it's hard to tell exactly (some differences in brightness are to be expected depending on the viewing angle) but it certainly looks much, much better than the XL2410T photos I've seen so far.
The purple blacks you mentioned are probably just caused by poor viewing angles of the TN panel. If you move your head it should go away. I can notice something similar with my G2400WD: the black at the far left or right side of the screen can appear purplish or yellowish at times (in a dark room), but it's just the viewing angle, the backlighting itself is uniform.
If you look around at the other 120Hz displays you should quickly notice it's got a little backlight bleed. 120Hz displays need to be brighter cuz of the 3D vision glasses and therefore you're gonna get more BLB overall on 120Hz displays (doesn't mean every1 has to have a lot of BLB though). Don't compare BLB to your standard 60Hz, compare it to the other 120Hz displays. Most of you would never use anything close to 100 brightness setting on a display like this or you'd get eye-fatigue or headache, for many even 50 would be too much to use so then I doubt the BLB will be noticable at all, the person that posted the BLB pics actually uses 100 brightness LOL and said he had not noticed any backlight bleed when he used it normally in a lit-up room.
I don't always use my monitor in a lit room and any amount of backlight bleed is annoying when watching movies or videos with a 2.35:1 aspect ratio. I've also noticed I like to keep my monitor brightness higher than most other people. But it's possible the photos exaggerate the bleed so I'm going to keep my eyes open for reviews.