An overwhelming number of developers were updating apps for iOS 7. Of 575 valid responses, 545 developers indicated that they were working on an update for iOS 7. That’s an adoption rate of 95%!
From what I've seen (and heard) so far, it looks like releasing new, paid, separate versions of apps for iOS 7 will be a common trend among developers. I think that, in most cases, it makes sense considering the major rewrite and redesign required by iOS 7 to ensure an app can be technically and visually ready by this Fall.
If we'll end up with an App Store full of old iOS 6 apps kept for “compatibility mode” or existing customers, I believe properly showcasing iOS 7 apps will be even more necessary in the (already crowded and poorly searchable) App Store.