http://docs.python.org/library/string.html#format-specification-mini-language mentions:
Changed in version 2.7: Added the ',' option (see also PEP 378).

PEP 378 tells me:
The ',' option is defined as shown above for types 'd', 'e', 'f', 'g', 'E', 'G', '%', 'F' and ''.

However:

Python 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit (Intel)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>> '{:.2,%}'.format(.537568)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid conversion specification
>>> '{:2,%}'.format(.537568)
'53.756800%'