Update: Also see: http://www.cocoacontrols.com/
The best site is still here, and http://www.iphoneexamples.com/
Others are iosdevelopertips.com, iphonedevsdk.com, iphone.zcentric.com and, of course, the official Apple Dev forums.
I have met many (mainly barcamp) communities that begin such efforts, but did not manage to get through. I would agree that such a site would be useful, because there are many problems of cocoa touch-gifts that are not in OS X (memory, performance, background processes, etc.).
I can only come to the conclusion that everyone is under their employer / client NAD clients and / or everyone is too busy to create such a site!
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