I picked up an Onyx GBA SP at Electronics Boutique the day they started selling them and that particular model had a relatively large speck of dust about 2 or three pixels across although not very large from top to bottom. I took it back and asked the clerk if I could get it replaced. She was curious because she'd never really seen that sort of defect in one of the GBA SP's. So I showed her and, sure enough, she saw it too.
Anyhow, my new one is just fine. I think it may have a tiny little speck of something or other under the screen but I was really really worried about finding one when I was looking and I haven't really given it any thought since.
So I guess that some models do have legitimate problems with dust and the Onyx and Flame models are not impervious to this defect either. Not much you can do about it but hope you get a good one.