ooh pretty. nice vector! I love the flowers in Cagalli's hair.
the background looks fine. it has a nice mood, kinda like a dawn-type feeling. I think the simplicity works, because you can't see much of a background anyways (anything complicated would be covered up anyway). umm, maybe that slight lens-flare on Athrun's face can be taken out, it's not really needed.
one thing though, the image looks a bit stretched/flattened, esp Cagalli's face...