This is a block of code, which creates a Class for managing Undo chunking in Autodesk Maya, Python scripts. I have tested this in my Maya 2011 setup, and it works great. This code snippet came from Rob Galanakis, via Tech-artist.org forums: http://tech-artists.org/forum/showthread.php?2522-Undo-in-Maya-(Mel-Python-Execution)

maya_UndoContext.py

import maya.cmds as mc

###########################################################################
## This wraps commands in a maya UndoContext
# ------------------------------------------------------------------------- 
class UndoContext(object):
    def __enter__(self):
        mc.undoInfo(openChunk=True)
    def __exit__(self, *exc_info):
        mc.undoInfo(closeChunk=True)

## This is how you call to the UndoContext()
with UndoContext():
    #... your code here....