I'm also getting this error.

It appears to me to be caused by the length of the module name rather than case.
mp_bug.py will run fine for me, but if I change it to mp_bug_longname.py then I will get this error.

import multiprocessing
import time

class MP_Bug(multiprocessing.Process):
    def __init__(self):
        multiprocessing.Process.__init__(self)
    def run(test):
        for x in range(1, 10):
            print x
            time.sleep(1)

if __name__ == '__main__':
    p = MP_Bug()
    p.start()