Cannot index a dataframe with a boolean dataframe
Sorry if there is a specific reason why this is not supported, but with boolean dataframe indexing, I would expect the following to work:

http://pandas.pydata.org/pandas-docs/dev/indexing.html#indexing-a-dataframe-with-a-boolean-dataframe

```

In [36]: df_rand = pd.DataFrame(np.random.randn(10,10))

In [37]: df_rand < 0
Out[37]:
       0      1      2      3      4      5      6      7      8      9
0  False  False   True  False   True  False   True  False   True   True
1  False   True   True  False   True  False  False   True  False   True
2   True   True   True  False   True  False  False   True  False  False
3  False  False   True  False   True   True   True  False  False   True
4   True  False  False   True  False  False   True  False  False  False
5  False  False  False  False  False  False  False  False  False  False
6  False   True  False  False   True   True   True  False  False   True
7  False  False  False   True  False  False   True  False   True  False
8   True   True   True  False   True   True   True  False  False  False
9  False  False   True   True   True  False   True   True   True  False

In [38]: df_rand[df_rand < 0] = -0.1

In [39]: df_rand[df_rand < 0] += 0.1
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
C:\Python27\Scripts\<ipython-input-39-6dc58d9235e9> in <module>()
----> 1 df_rand[df_rand < 0] += 0.1

C:\Python27\lib\site-packages\pandas\core\frame.pyc in __getitem__(self, key)
   1655             return self._getitem_multilevel(key)
   1656         else:
-> 1657             return self._get_item_cache(key)
   1658
   1659     def _getitem_array(self, key):

C:\Python27\lib\site-packages\pandas\core\generic.pyc in _get_item_cache(self, item)
    479                 from pandas.core.frame import DataFrame
    480                 if isinstance(item, DataFrame):
--> 481                     raise ValueError('Cannot index using (boolean) dataframe')
    482                 raise
    483

ValueError: Cannot index using (boolean) dataframe

In [40]: print pd.__version__
0.8.0b1
```
