Thanks for the update.  Since the fix will be in a future version of OS X 10.7 Lion, and which has not been released yet, so it is not appropriate to change mmap until there has been an opportunity to test it.  But even then, we would need to be careful about adding a compile-time test as OS X binaries are often built to be compatible for a range of operating system version so avoid adding compilation conditionals unless really necessary.  If after 10.7 is released and someone is able to test that it works as expected, the standard way to support it would be to use the Apple-supplied availability macros to test for the minimum supported OS level of the build assuming it makes enough of a performance difference to bother to do so: http://developer.apple.com/library/mac/#technotes/tn2064/_index.html

(Modules/_ctypes/darwin/dlfcn_simple.c is one of the few that has this kind of test.)