http://docs.python.org/library/string.html#formatstrings
field_name        ::=  (identifier | integer) ("." attribute_name | "[" element_index "]")*
element_index     ::= integer

Subsequent text indicates __getitem__() is used but does not overtly say that a string can be used; but
http://docs.python.org/whatsnew/2.6.html#pep-3101-advanced-string-formatting
gives the example
>>> 'Content-type: {0[.mp4]}'.format(mimetypes.types_map)
and clearly '.mp4' is passed to __getitem__(); a string, not an integer.

Clearly one of these is wrong !
Given that the "what's new" doc goes into some detail about how the content of [...] gets parsed, I'm guessing it's right and the grammar is wrong.