If this will work on Android, too, and I believe it will, it could be an instant win for HTML5 app developers, because Facebook's webstore would be the first such "cross-platform" store where you can showcase your mobile webapp.
If they keep it iOS only, I don't see much incentive to build a lower performance webapp instead of a native app, unless you already have a html5 app and you want to use their webstore to showcase it there.
If they keep it iOS only, I don't see much incentive to build a lower performance webapp instead of a native app, unless you already have a html5 app and you want to use their webstore to showcase it there.