Luckly, the Retina display in the iPhone 4 and the iPad both have 2x resolution of the original iPhone - meaning you only need two sets of graphics (one at regular resolution, one at 2x resolution) to work on all 3 devices. Where would a 7" iPad fit in this?
I'd guess that a 7" iPad would either share the same resolution as the iPad 9" OR the resolution of iPhone 4.
Either would be possible and would mean developers wouldn't have to do much more work.