from this link [http://en.wikipedia.org/wiki/PARI/GP#Usage_examples], I wanted to contrast arbitrary precision with the other pgm I use, Pari/GP.  I tried the xample there which was:
123456! + 0.
Now, behavior seems the same without the "+0." for both.  However, while Pari returns the answer shown on link quickly, Python after a delay returned an error, related to float conversion.  Here is the progression from a smaller number to the problem:
>>> math.factorial(12) +0.
479001600.0
>>> math.factorial(123) +0.
1.214630436702533e+205
>>> math.factorial(1234) +0.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to float
>>>

Note that while Python's long type gives you unlimited-size integers, the float type doesn't make such promises: it is just a double-precision float.  As such, math.factorial(1234) cannot be interpreted; it would simply be positive infinity.

Right; this is expected behaviour.  The error you're seeing comes from the implicit conversion of 1234! from long to float.