This is a known bug with the iOS 4.1 SDK and building using LLVM for the iPhone Simulator. You can read all about it in this thread on Apple’s Developer Forums.
The recommended solution is to add the following to Other C Flags
in your project’s build settings: -D__IPHONE_OS_VERSION_MIN_REQUIRED=040100
where you replace 040100 with your deployment target version (030000 for 3.0, for example).