As you can't currently submit apps that have been built for iOS7 it shouldn't have an impact for developers working on rewrites. The biggest problem being the prerelease documentation is down whilst the current docs are up, however as they're included with XCode that's actually not a huge deal.

Plus iOS7 would be on a tight schedule if Apple had actually announced a release date, as it is iOS7 is on a schedule we just don't know what it is.