Unexpected Behavior: Assigning ndarray to variable results in new object
The below code should yield True after the assignment, but instead it yields False.  This happens in Python 2.7.3 / Pandas 0.12.0 / Numpy 1.7.1.  This means that any changes to the ndarray 'x' are not guaranteed to be reflected in the source data frame 'df', which is not normal Python behavior for assignments of mutable objects.

``` python
>>> import pandas
>>> df = pandas.DataFrame({'a':[1,2,3,4,5],'b':[2,4,6,8,10]})
>>> x = df['a'].values
>>> x is df['a'].values
False
```
