Will iOS apps that are just UIWebView's be rejected?

As per section 4.2 of the App Store Review Guidelines, yes, this kind of app will be rejected.

4.2 Minimum Functionality
Your app should include features, content, and UI that elevate it beyond a repackaged website. If your app is not particularly useful, unique, or “app-like,” it doesn’t belong on the App Store. If your App doesn’t provide some sort of lasting entertainment value, it may not be accepted.

@Moshe is correct with pointing out what they SAY, but what they do is a completely different story as we all know. I think it depends on the reviewer that you get, and the mood they are in! Take a look at this example.

KSL app is just their mobile website wrapped in a Web view with a back and forward button, and its in the store.

Their mobile site: http://m.ksl.com/

Their App Link: http://itunes.apple.com/us/app/ksl.com/id363278608?mt=8

Update: KSL now has an updated app, which is an actual app... so my example no longer pertains to them.