MLton runs fine on Darwin (and on Mac OS X).
Executables that save and load worlds on Darwin 11 (Mac OS X Lion) and higher should be compiled with -link-opt -fno-PIE ; see MLtonWorld for more details.
ProfilingTime may give inaccurate results on multi-processor machines. The SIGPROF signal, used to sample the profiled program, is supposed to be delivered 100 times a second (i.e., at 10000us intervals), but there can be delays of over 1 minute between the delivery of consecutive SIGPROF signals. A more complete description may be found here and here.