Further details:

- The original test case I'd submitted crashed on the development branch of NetBSD as well as Mac OS X Snow Leopard, but not the most recent stable branch of NetBSD.  I've found a separate test case that crashes on both branches of NetBSD, but not OS X...  This is quite possibly a separate bug, but the means of triggering it is directly related, so I'm including it here.

- I also built Python 2.7.2 under Solaris to see if either test case resulted in a crash there, and they do not, so it seems this is BSDish somehow (or else, the Mac OS X and NetBSD crashes are two separate bugs).

- With NetBSD, I also created tests in C that use the Expat library directly, submitting the very same test data, and they do not crash, they return the expected results, so it appears there's definitely something happening in Python somewhere that's causing this.

This is the (non-debug) crash trace from the separate NetBSD test.  (I will look at building a debug version of Python when I get a chance...)  I'm running Python 2.6.7 on the NetBSD machines.

#0  0xbb93ff64 in XML_ParserCreate () from /usr/X11R7/lib/libexpat.so.1
#1  0xbb9348a3 in XML_GetCurrentLineNumber () from /usr/X11R7/lib/libexpat.so.1
#2  0xbb956743 in set_error () from /usr/pkg/lib/python2.6/site-packages/pyexpat.so
#3  0xbb956d21 in xmlparse_Parse () from /usr/pkg/lib/python2.6/site-packages/pyexpat.so
#4  0xbbb048b0 in PyCFunction_Call () from /usr/pkg/lib/libpython2.6.so.1.0
#5  0xbbb5a3d7 in PyEval_EvalFrameEx () from /usr/pkg/lib/libpython2.6.so.1.0
#6  0xbbb5add8 in PyEval_EvalCodeEx () from /usr/pkg/lib/libpython2.6.so.1.0
#7  0xbbb5914e in PyEval_EvalFrameEx () from /usr/pkg/lib/libpython2.6.so.1.0
#8  0xbbb5add8 in PyEval_EvalCodeEx () from /usr/pkg/lib/libpython2.6.so.1.0
#9  0xbbb5ae22 in PyEval_EvalCode () from /usr/pkg/lib/libpython2.6.so.1.0
#10 0xbbb72f12 in run_mod () from /usr/pkg/lib/libpython2.6.so.1.0
#11 0xbbb72fb5 in PyRun_FileExFlags () from /usr/pkg/lib/libpython2.6.so.1.0
#12 0xbbb745e4 in PyRun_SimpleFileExFlags () from /usr/pkg/lib/libpython2.6.so.1.0
#13 0xbbb74ce5 in PyRun_AnyFileExFlags () from /usr/pkg/lib/libpython2.6.so.1.0
#14 0xbbb80322 in Py_Main () from /usr/pkg/lib/libpython2.6.so.1.0
#15 0x080487e9 in main ()