At Loopt we tried to do this. We made a hybrid app (Loopt Star), using web views for content and native code for tabs, Facebook Connect, modal dialogs, animations, etc. It didn't work. WebViews/Safari basically don't cache anything at all, and javascript performance is awful.
Out of desperation we even tried including a reverse proxy as part of the app to force meaningful image and css caching. Yes, the webviews were serving content from localhost, which was proxied transparently to our real servers and cached aggressively. Even that wasn't enough.
If you want your app to have a first class user experience, you have to go native. Maybe in the future things will improve, but right now users can tell.
Out of desperation we even tried including a reverse proxy as part of the app to force meaningful image and css caching. Yes, the webviews were serving content from localhost, which was proxied transparently to our real servers and cached aggressively. Even that wasn't enough.
If you want your app to have a first class user experience, you have to go native. Maybe in the future things will improve, but right now users can tell.