I should have posted a follow-up long before now, so my apologies. Some of my earliest postings here were on accessing Web Services from the iPhone. At the time I wrote that posting, I had not yet been accepted into the iPhone developer program, and wasn't aware that CoreServices.framework existed in the simulator, but not on the device.
I got accepted into the program and discovered the problem just before WWDC, and spent a good part of that conference trying to corner somebody from Apple about it. I was not able to get anyone who was even aware of the discrepancy, let alone somebody who could tell me whether the framework was intentionally left off of the device or if it would ever be brought over. The best I got was a very non-committal, off-the-record statement that WebServicesCore probably "wasn't long for the world". It hasn't been updated for a few years, and probably won't be. REST is the way of the future, why bother with SOAP, seems to be the feeling.
Of course, there is a huge installed base of SOAP web services which, I would argue, is a good enough reason to update and expand WebServicesCore. More importantly, many of the people coming to the iPhone are coming from .NET, which has extremely robust support for SOAP web services. I would argue that not having a good SOAP client in Cocoa Touch or Foundation makes our platform look bad.
Anyway, if you do need to access SOAP web services from the iPhone, you're stuck rolling your own or looking for a third-party SOAP library. There is an excellent thread over at iPhoneDevSDK. Right now, that's the best source of information I have to offer anybody about SOAP web services.
Now, if you're interested in RESTful web services, Apple has an article on that. In fact, they have an article that I co-wrote.