We use a gray null texture background sprite to match our loading image background and have it so that the aspect is the same as the launch image assigned by xCode. It works pretty nice and you can only notice the change in resolution ever so slightly. You probably wouldn't notice at all if you could shut off the launch image fading on iOS.
Either that or stitch two images together, with pixel snapping on you should not be able to see the seams.
You can test it out in the iOS simulator and fairly quickly see the results.
I'd stick with the 1024x1024 if you want to support all devices. Plus you can't really select device compatibility with iOS so if you go larger and someone downloads on a 3GS they'll have a bad time.
I do with AppGameKit had commands to check OpenGL compatibilities. We found out that there are a few devices out there that have OpenGL support but have remove support for mipmapping. There's no way for AppGameKit to detect this and not use mipmapping if it isn't supported which leads to no image showing at all...
