When I build my project (any project, really - I tried creating a new empty project with the same results), it builds fine with GCC 4.2 under either Xcode4 or Xcode 3.2.4.
If I build using LLVM 2.0 under Xcode4 or with LLVM 1.5 under Xcode3, I get compile-time build failures, but only when building for the Simulator.
The build errors that I get under LLVM are all in headers over which I have no control, such as UIView.h, UIDevice.h, UIApplication.h, UITextView.h and UIWebView.h in UIKit and CGPDFContext.h in CoreGraphics.
Here's an example error, in WebView.h:
@property(nonatomic) UIDataDetectorTypes dataDetectorTypes __OSX_AVAILABLE_STARTING(__MAC_NA, __IPHONE_3_0); //<--- Unknown type 'UIDataDetectorTypes'
when UIDataDetectorTypes is indeed declared in UIDataDetectors.h which is included in that file.
sniff