Mac OS X 10.2 includes a new General/WebServicesCore Framework (which is part of the General/CoreServices Framework) which allows one to make XML-RPC calls from Cocoa apps. (And Carbon apps, too, by the way.)
General/SimpleObjectAccessProtocol is also used somehow - must read up!
Ranchero has a demo - http://ranchero.com/cocoa/xmlrpcdemo/
Also there are examples in file:///Developer/Examples/Web%20Services/
also see here for a directory of Web Services:
I wrote a short article on using Apple’s Web Services Core to connect to Flickr’s XML-RPC API: http://compsigh.com/main/article/2
There is an open source app, WSDL2ObjC, that attempts to replace WS-Core altogether: http://code.google.com/p/wsdl2objc/