This is happening because if you pass an object instead of a string as the first argument in a logging call, it's treated as a message object whose __str__() will be called to get the actual message when it's needed, as documented here:

http://docs.python.org/howto/logging.html#arbitrary-object-messages

If, instead, you do it like this:

# coding: utf-8
import logging

logging.basicConfig()
try:
    raise Exception(u'ą')
except Exception:
    logging.getLogger('general').exception(u'An error occurred')

you get

ERROR:general:An error occurred
Traceback (most recent call last):
  File "bug_12339.py", line 6, in <module>
    raise Exception(u'ą')
Exception: \u0105