So far I haven't been able to find the specifications for the iPhone screenshots we'll submit along with our application. (It's universal, so I'm also taking iPad screenshots.) Can they / should they be of iPhone 4 resolution (640x960 instead of 320x480)?
They can, and should, be iPhone 4 resolution (640x960). You will be able to see the specifications when you create a new version, and click the "?" under Edit images.