ValueError when slicing with NaN values
this didn't fail before:

In [30]: a = DataFrame([[1,2,3],['b',None,'a'],[1,'b',9]])

In [31]: a
Out[31]: 
   0     1  2
0  1     2  3
1  b  None  a
2  1     b  9
## In [32]: a[a[1]=='a']

ValueError                                Traceback (most recent call last)
/Users/jorge/<ipython-input-32-3a0cb4ca7415> in <module>()
----> 1 a[a[1]=='a']

/Users/jorge/.python_unstable/lib/python2.7/site-packages/pandas-0.7.2-py2.7-macosx-10.7-intel.egg/pandas/core/frame.pyc in **getitem**(self, key)
   1422 
   1423             # also raises Exception if object array with NA values

-> 1424             if com._is_bool_indexer(key):
   1425                 key = np.asarray(key, dtype=bool)
   1426             return self._getitem_array(key)

/Users/jorge/.python_unstable/lib/python2.7/site-packages/pandas-0.7.2-py2.7-macosx-10.7-intel.egg/pandas/core/common.pyc in _is_bool_indexer(key)
    321         if not lib.is_bool_array(key):
    322             if isnull(key).any():
--> 323                 raise ValueError('cannot index with vector containing '
    324                                  'NA / NaN values')
    325             return False

ValueError: cannot index with vector containing NA / NaN values

Note: With integer values the exception is not raised:

In [34]: a = DataFrame([[1,2,3],['b',None,'a'],[1,2,9]])

In [35]: a
Out[35]: 
   0   1  2
0  1   2  3
1  b NaN  a
2  1   2  9

In [36]: a[a[1]=='a']
Out[36]: 
Empty DataFrame
Columns: array([0, 1, 2])
Index: array([], dtype=int64)
