I have the same thing on my black 64GB 3G in the exact same spot (under the headphone jack). It's either the design or something in the production process that's causing this since everyone seems to be having the issue in the exact same location. Honestly, I would've never noticed it if not for this thread (and trust me, I've inspected my iPad 2 with a fine-toothed comb), although it is much easier to see on the white iPads. I think the new super-thin bezel and the glued (rather than clipped) screen are the likely culprits here.
I certainly don't plan on exchanging mine for this reason alone, particularly since I'd probably just get one that's worse in some other way, and there's nothing loose or clicking on mine. My iPad does have a bit of minor backlight bleed, but since I got it at Target, I figure I have 90 days until I need to return it. So, I'm going to wait it out a little longer until iPad 2's are easier to find and then I'll swap mine out. Hopefully these manufacturing bugaboos will be taken care of in the next couple of months.
Despite the couple of very minor issues my iPad has, I'm really happy with it. Admittedly, it does help knowing that I have the option to return it in 90 days, but I'd be satisfied even if that weren't the case.