I know it's too early for burn-in but some screens (might I say... all?) come from the factory with some sort of flaw. None of them are perfect and most have flaws so imperceptible that most people wouldn't notice. Every nexus S and Galaxy I've seen had some sort of area or line or multiple lines on the screen which were a bit different color, not as bright, or something along those lines. Some are worse than others, and I noticed the flaws right after taking it out of the box brand new. (I've been through about 6 Nexus S's between me and a few friends).
It's not really the same as a traditional LCD/LED screen since there's not a big backlight. Each pixel is individually lit and some may have specifications that are just on the edge of within manufacturer limits. There's almost 400,000 pixels on the screen... the chances of all of them being created identically is slim to none.
If it bothers you I say just take it back and get an exchange, showing them the problem area on the screen. Just hope your new one will be "less flawed" =)