I have been trying to generate code coverage data for my application on XCode 4.2.1 (build 4D502) using the steps outlined at http://code.google.com/p/coverstory/wiki/UsingC
It doesn't work with llvm-gcc; you'll need to use the "LLVM Compiler 3.0" (aka clang). (I assume by the 'llvm-gcc' tag that you're not currently using it.)
I did this just last week, so it's definitely working.