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On OS X (10.6) with Python 2.6 
import plistlib
plistlib.writePlistToBytes(dict())
AttributeError: 'module' object has no attribute 'writePlistToBytes'
Python documentation contains no information regarding when writePlistToBytes was add. http://docs.python.org/dev/library/plistlib.html#plistlib.writePlistToBytes 

You are looking at the documentation for Python 3, not Python 2. For Python 2.6, you should refer to its documentation: http://docs.python.org/release/2.6.6/library/plistlib.html
For Python 3, many references to strings were changed to bytes as part of the changing of Unicode strings and 8-bit strings in Python 2 to strings and bytes in Python 3. This was one of them.