Cocoa is not really the foundation of web applications, so I would not recommend you go that route. WebObjects (see KiwiBastard's answer) used to be written in Objective-C, but (since version 5, I think) is written in Java.
OS X UNIX, - ( , ). , CGI perl, python, C . JBoss ( Java) OS X Server - Java.
, RoR PHP, - python 1, TurboGears, Django, Zope ..
1 Python 2.5 OS X 10.5