With attached x.py:

$ cat x.py
"""
>>> foo()
"""
def foo():
    1/0

if __name__ == '__main__':
    import doctest, pdb
    try:
        doctest.testmod(raise_on_error=True)
    except doctest.UnexpectedException, e:
        pdb.post_mortem(e.exc_info[2])

$ ./python.exe x.py
> /Users/sasha/Work/python-svn/trunk/x.py(5)foo()
-> 1/0
(Pdb) up
> <doctest __main__[0]>(1)<module>()
(Pdb) l
[EOF]


Apparently the problem is that the default getlines gets restored in the  
"finally:" section in DocTestRunner.run() before post_mortem is invoked.

The only solution that comes to mind is to patch pdb.post_mortem 
similarly to the way pdb.set_trace is patched but not restore it until 
the end of testmod().

Am I missing a simpler solution?

#!/usr/bin/env python
# This does what you want. Change pydb for pdb if you prefer it.
import pydb
import doctest
import sys
sys.excepthook = pydb.exception_hook
try:
    doctest.testfile("story.txt",verbose=False,raise_on_error=True)
except doctest.UnexpectedException, failure:
    pass
exc_info = failure.exc_info
raise exc_info[0], exc_info[1], exc_info[2]

#!/usr/bin/env python
# Slight mistake in last post
import pydb
import doctest
import sys
sys.excepthook = pydb.exception_hook
try:
    doctest.testfile("story.txt",verbose=False,raise_on_error=True)
except doctest.UnexpectedException, failure:
    exc_info = failure.exc_info
    raise exc_info[0], exc_info[1], exc_info[2]

I think this is just too complicated to implement in a sane way.  doctest+pdb is just a bit too hacky to be fully supported.