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That is a tricky line, indeed. Assuming http://photos.appleinsider.com/App%20Store%20Review%20Guidel... is the real deal, Apple tries to do it with the following rules:

2.20: Developers "spamming" the App Store with many versions of similar apps will be removed from the iOS Developer Program

12.4: Apps that are simply web clippings, content aggregators, or a collection of links, may be rejected

Notice the 'may' in that last clause.

Sadly, I don't think Apple enforces these guidelines very much — the top three (by number of apps) developers in the iOS App Store are [1]:

A lot of the apps for each of these companies are very similar or are basic web clips.

1 — Source: iTunes Enterprise Partner Feed dump from a month or so ago.