Apple has capriciously pulled apps from their app store too many times to count, so I assume you specifically mean when have they pulled competing apps because they decided to introduce their own apps with similar features. Well, they've done that numerous times, too. For instance:

http://www.tuaw.com/2011/04/08/quickpick-update-in-limbo-for...

http://appleinsider.com/articles/10/06/02/apple_yanks_widget...

EDIT: fixed first link