Update
Just got back from my Genius appointment.
The Genius--an affable and genuinely helpful dude--struggled at first to see the issue with my display under the bright lights of the Apple Store. But after taking it to the "infirmary" in the back and showing it to the technicians, they all agreed it was definitely "at the far end of what Apple normally considers an acceptable variance in brightness" and that it was worth replacing.
Nice to get confirmation from an Apple tech that I wasn't being entirely crazy/neurotic about this issue!
So I'll be taking my 5-day-old computer in for surgery on Friday.
My logic is as follows: if a tech acknowledges this issue and sees what my current display looks like (and what my standards for a "good" display are), he'll make doubly sure that I don't wind up with a worse display. Hope I'm right.
And in any case, the Genius reassured me that I would still be able to return the MBP for an exchange or refund if the screen replacement proved unsatisfactory. And if it comes to that, at least I'll have a better argument for having my replacement computer "checked out" at the factory before it's shipped to me. (I genuinely hope it doesn't come to that, though.)
Will keep you posted!