The background:
I have a project that I last built on 10.5 on a PPC computer using xcode v3.1. It builds against the 10.4 SDK. I now have a MacBook with 10.6 on it and Xcode v3.
If anyone gets this when building a C command line app in Xcode and is experimenting with cutting edge settings: I got this very same warning when I switched to Clang and enabled Link-Time Optimization (LLVM_LTO = YES) just for the fun of it.