Playing with PyInstaller I have found that the final part of 
_elementtree.c:

Index: Modules/_elementtree.c
===================================================================
--- Modules/_elementtree.c      (revisione 59540)
+++ Modules/_elementtree.c      (copia locale)
@@ -2780,7 +2780,10 @@

       );

-    PyRun_String(bootstrap, Py_file_input, g, NULL);
+    if (PyRun_String(bootstrap, Py_file_input, g, NULL) == NULL)
+        return;

    elementpath_obj = PyDict_GetItemString(g, "ElementPath");

execute a bit of python code without checking the return value.
That can lead to weird things playing with import hooks,
for example an assert like this can fail:

Index: Lib/test/test_elemettree.py
===================================================================
--- Lib/test/test_elemettree.py (revisione 0)
+++ Lib/test/test_elemettree.py (revisione 0)
@@ -0,0 +1,21 @@
+#! /usr/bin/env python
+
+def importHook(*args, **kwargs):
+    if 'xml.etree' in args:
+        raise ImportError
+    else:
+        return __real__import__(*args, **kwargs)
+
+import os
+import __builtin__
+__real__import__ = __builtin__.__import__
+__builtin__.__import__ = importHook
+
+try:
+    import xml.etree.cElementTree as cET
+except ImportError:
+    pass
+else:
+    out = os.popen("python -c 'import xml.etree.cElementTree as cET; 
print dir(cET)'").read().strip()
+    assert str(dir(cET)) == out, (str(dir(cET)), out)
+

Fredrik, can you take a look?

This is fixed in the ET 1.3-compatible codebase.  Since it's too late to
add ET 1.3 to 2.6, I guess it's time to make a new 1.2 bugfix release
for 2.6.

At this point, this affects 2.5, 2.6, and 3.1 (and the normal 1.0.5
release of cElementTree); what's required to get this fixed and queued
up for micro/minor releases?

Sidenote, the patch posted above still leaks a reference-

For 2.5, this will not be fixed, as it is not security-critical.

Note that "fail silently" is a bit of a misnomer - if the embedded import 
doesn't work, portions of the library will fail pretty loudly.  Feel free 
to use some variation of the suggested patch, or just wait until the next 
upstream release gets imported (if ever).

Fixed with latest xml.etree.