In researching iOS &OS X declares, I found this article (having a link to another series of brillant articles) about the Objective C Runtime framework in both systems. It makes a really good read and gives some insight into the internal method handling (and why OS X 64bit apps will be a lot faster because of changes in the new 64bit objC methods): http://cocoasamurai.blogspot.de/2010/01/understanding-objective-c-runtime.html?m=1
The other article series mentioned inside starts here: http://www.friday.com/bbum/2009/12/18/objc_msgsend-part-1-the-road-map/