You should not be using automatic reference counting. That is for Objective-C code.
EDIT: On clean and rebuild I've discovered SDLMain is throwing up a ton of errors related to automatic reference counting mode. I'm going to assume I should be compiling with automatic reference counting OFF -- could someone confirm this for me?
You may have a search path problem, where Xcode can't find the SDL framework and/or headers. Start with adding the path to SDL to Xcode's Framework Search Paths build setting. If that doesn't work, add a path to the Header Search Paths build setting. If you're using Xcode 4, the following article may help you:
Using SDL with Xcode 4