We've found that the iOS client generates hundreds of warnings when compiled in a project on Xcode 4.2.1 using the Apple LLVM compiler (when using the GCC compiler, it is fine). The Apple LLVM compiler seems stricter with its warnings.
We can hide these errors with a compiler flag set on the ES implementation files, but if the project also uses the new ARC memory management system, then a different set of warnings that comes up that we've not been able to suppress. (We are using the flags "-fno-objc-arc -w" on all the ES files.)
None of this prevents running the code (as far as we have seen) since these are just compiler warnings. But having hundreds of warnings from ES can obscure real problems in our own code.