<p>Is there any centralized repository of useful Objective-C / Cocoa libraries as there is for Perl, Ruby, Python, etc.?</p>

<p>In building my first iPhone app, I'm finding myself implementing some very basic functions that would be just a quick "gem install" away in Ruby.</p>

Unfortunately not :(
There are some very useful sites however.  I find one of the best is
cocoadev.com
as it contains lots of useful information about many of the more obscure classes usually including snippets of code to do some really cool things :)
Maybe we (the cocoa community) should look into building something like this!
Oh and I just remembered this site
cocoadevcentral.com
which is also very good for starting out with cocoa.