“_vproc_transaction_begin”, referenced from:
___gcov_init in libgcov.a(_gcov.o)
_vproc_transaction_begin$non_lazy_ptr in libgcov.a(_gcov.o)
“_vproc_transaction_end”, referenced from:
_gcov_exit in libgcov.a(_gcov.o)
_vproc_transaction_end$non_lazy_ptr in libgcov.a(_gcov.o)
ld: symbol(s) not found

Turns out (http://lists.apple.com/archives/xcode-users/2009/Sep/msg00066.html), the active SDK has to be the same as the current OS; setting it to 10.6 resolved the problem.

I added a bit of text processing to make the output a little more readable:

find “${OBJROOT}” -name *.gcda -exec gcov -o “${OBJ_DIR}” {} \; | egrep “^File|^Lines” | sed -E “s@File ‘$SRCROOT/@@;s@(\.[a-zA-Z])’@\1: @;s@Lines executed:([0-9.%]+) of ([0-9]+)@\1 (\2)@” | paste -d” ” – - | sed -E “s@^([^:]+):([^(]*)(\([^)]+\))@\2:\1\3@” | sort -n | sed -E “s@^([^:]+):([^(]*)(\([^)]+\))@\2:\1\3@”

Rather complicated looking command line, but it produces output like the following, sorted by coverage percentage (lowest first):

Classes/TSClassFile.m: 0.82% (243)
Classes/TSSomeOtherFile.m: 0.89% (676)