Sorry, still there:

Python 2.7rc2 (r27rc2:82137, Jun 26 2010, 11:27:59) 
[GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mimetypes
>>> mimetypes.guess_extension('image/jpeg')
'.jpe'
>>> mimetypes.init()
>>> mimetypes.guess_extension('image/jpeg')
'.jpeg'

The fact that it's not reproducible on other Linux systems (I can't reproduce on the RedHat box I have to hand) might suggest there's something odd about Debian's mime.types . But I've just tried it passing init() the mime.types from the (working) RedHat box, and it's still producing the odd behaviour. (And I'm now on Debian 5.0, so it's not a Debian 4.0-specific issue either.) Wish I had a convenient Ubuntu install to try it on.

Bizarre.