>>> itertools.repeat.__doc__
'repeat(object, times=None)\nReturns an iterator which returns the object the specified number of times.\n\nIf times is None, returns the object endlessly.'
>>> itertools.repeat('a', times=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __new__() argument 2 must be (unspecified), not None

Also, maybe it's a good idea to add case for None and negative times in itertools.repeat unit test. You can get inspiration from http://bugs.python.org/issue19145