WebServicesCore

Mac OS X 10.2 includes a new WebServicesCore Framework (which is part of the CoreServices Framework) which allows one to make XML-RPC calls from Cocoa apps. (And Carbon apps, too, by the way.)

http://www.xmlrpc.com/

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:

http://www.xmethods.com/

Apple's docs:

http://developer.apple.com/documentation/InternetWeb/WebServices-title.html

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/