Can you try importing pyplot from a regular python prompt?  If that
works, then the issue is with ipython messing up the import process of
mpl.

 
I get exactly the same problem...
 

laserson@hobbes:~$ python
Python 2.7.1 (r271:86832, Dec 13 2010, 14:21:05) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>> import matplotlib.pyplot
RuntimeError: module compiled against ABI version 2000000 but this version of numpy is 1000009
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/laserson/matplotlib/lib/python2.7/site-packages/matplotlib/pyplot.py", line 23, in <module>
    from matplotlib.figure import Figure, figaspect
  File "/Users/laserson/matplotlib/lib/python2.7/site-packages/matplotlib/figure.py", line 16, in <module>
    import artist
  File "/Users/laserson/matplotlib/lib/python2.7/site-packages/matplotlib/artist.py", line 6, in <module>
    from transforms import Bbox, IdentityTransform, TransformedBbox, TransformedPath
  File "/Users/laserson/matplotlib/lib/python2.7/site-packages/matplotlib/transforms.py", line 34, in <module>
    from matplotlib._path import affine_transform
ImportError: numpy.core.multiarray failed to import
>>>