While the behavior cannot be reproduced in the trunk, in can be 
reproduced in the 2.6 release:

$ python -W ignore
Python 2.6.1 (r261:67515, Dec  6 2008, 16:42:21) 
[GCC 4.0.1 (Apple Computer, Inc. build 5370)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.pack('L', 'foobar')
'\x00\x00\x00\x00'
>>> struct.pack('I', 'foobar')
'\x00\x00\x00\x00'
>>> struct.pack('i', 'foobar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: required argument is not an integer
>>>