Thanks for letting us know on this.
I went to my local Fry's and picked up a clear Snap Shield Secure (unfortunately, for MSRP $35, but that's the instant gratification tax).
Definitely pricier than the generic ones online, but I have a 30 day return policy if the New iPad ones come out quickly, and I love the retention strip for the Smart Cover magnet.
Re: fit, I tried on my iPad 2 to get an ideal, then on my New iPad.
It's harder to snap in on the New iPad, though secure when you get there.
There is -very- slightly more bowing in the back (a fraction of a millimeter), which is only obvious when you look at the Apple logo/text on the back and press down--it gets slightly clearer.
In regular use I don't notice that at all. It doesn't creak, and the gap is so subtle that I can only -barely- feel it in the center, and not at all on the outer 2/3 of the device where your hands always go. On an opaque Shield, I don't think you'd realize there's a fit difference.
Also, I use the word "more" deliberately. The Shield bowed very slightly on my iPad 2 as well, though I don't think quite as much. I'd be willing to believe the amount of bowing on here would be within manufacturer tolerance even for a Shield used on the intended unit.
The ports are still all centered as far as I can tell. The camera looks a hair higher in the circle, but it's small enough that I'm not actually sure that it is. It's definitely not obscured.
I'll put it this way: it's good enough that I may or may not go to one fitted "correctly" when they come out. The only reason to would be if the extra thickness overstresses the snap-on lips. Otherwise the benefit would be minimal.
Edit: One last-minute observation. The bowing does make the top center edge of the Shield lift very slightly away from the tablet (again, fraction of a millimeter).
This has a couple of effects: the primary is to make the sleep button a little harder to hit; OTOH, if you have a smart cover, you never end up using that button.
The other worries me slightly more; there's a little gap beneath the cutout around the microphone, and dirt could get in there. So that would be the argument for finding an iPad 2-wielding friend and swapping this out later. I think I'll have an idea in a week or two whether it's really getting anything in there or not though.