I don't see any problem with taking it back again, though it really depends on how high your tolerance is for returning products repeatedly. Maybe you can go to different stores in the same chain...
I've purchased 4 DS systems for one reason or another, and 2 of them have had pixel problems. One was from an online importer that expressly forbade returns due to dead pixels, so I was stuck with that one. But the resolution of the DS is lower than that of the PSP, so I wouldn't tolerate even a single defective pixel.
For comparison's sake, I've bought 3 PSPs, and all three have had at least 3 stuck pixels. Because the resolution is finer, a few pixels don't bother me that much, but I got one recently that has a small cluster of them, so it's going back.