The Mac Roman encoding comes into play, because _commit opens _dirfile 
without explicitly specifying an encoding. io.open then gets the 
encoding via locale.getpreferredencoding, which returns mac-roman:

Majestix:Python-3.0rc3 martina$ 
DYLD_FRAMEWORK_PATH=/Users/martina/Downloads/Python-3.0rc3: ./python.exe 
-c "import locale;print(locale.getpreferredencoding())"
mac-roman

Two issues:
- since dumb.py handles encoding explicitly, shouldn't it specify the 
encoding for _dirfile as well? (or use a binary file; but this could 
cause new line-ending troubles...)
- is mac-roman really the appropriate choice for 
locale.getpreferredencoding? This is on Mac OS X 10.5, not Mac OS 9... 
The preferred encoding for Mac OS X should be utf-8, not some legacy 
encoding...

Seems to be related to r67310, which was intended to fix issue #3799
http://svn.python.org/view/python/branches/py3k/Lib/dbm/dumb.py?
rev=67310&r1=63662&r2=67310