from xml.etree import ElementTree as ET
from StringIO import StringIO
ET.parse(StringIO('<test/>'))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Program Files\IronPython 2.6 for .NET 4.0\Lib\xml\etree\ElementTree.py", line 862, in parse
File "C:\Program Files\IronPython 2.6 for .NET 4.0\Lib\xml\etree\ElementTree.py", line 581, in parse
File "C:\Program Files\IronPython 2.6 for .NET 4.0\Lib\xml\etree\ElementTree.py", line 1117, in init
ImportError: No module named expat; use SimpleXMLTreeBuilder instead

 
ElementTree 1.2.7 preview available at [1] has this comment: "ElementTree 1.2.7 preview is now available. This is 1.2.6 plus support for IronPython.". We've also used this version with Robot Framework [2] and at least in our relatively simple use it works great.
 
It might be worth checking how ET 1.3 alpha 3 works too. It's newer than 1.2.7 so it could have same IronPython fixes. CPython 2.7 uses ET 1.3 [3] so this would make IPY 2.7 API compatible with CPython 2.7 in this regard.
 
Notice that there's already work item #21407 about adding _elementtree module for the current ET implementation.
 
[1] http://effbot.org/zone/element-index.htm
[2] http://code.google.com/p/robotframework/wiki/DotNetSupport
[3] http://docs.python.org/library/xml.etree.elementtree.html