I've been waiting for Apple to implement this themselves- it always seemed odd that they didn't. Especially back in the 'web apps are the iPhone API' days.
It's nice to see this hole closed, although still relying on the app store for distribution is a shortcoming. An even better option would be a third party browser that could support these extensions for any site, but then again, that would be duplicating Apple's built in functionality, and chances are they wouldn't approve the app for inclusion in the store.