Kevin at Hot Cocoa has done a splendid tutorial series on accessing web services with Cocoa. He chooses Twitter as his example, which is a nice simple service to get started with. And starting from an empty project, his videos show how to:
1) Make http requests from the server
2) Parse the XML result in a very easy way using XPath.
3) Use the OS X Keychain for storing your webservice password.
Together with all the usual Cocoa tutorial stuff of linking buttons to actions and tles to datasources.
Highly recommended.
Posted by stevewaddicor 
OS X Notebook 