Different types returned for numpy 1.5.1 and 1.6.1
I noticed that certain `numpy` functions return `pandas.Series` in 1.5.1 and `numpy.array` in 1.6.1.  This makes it hard to write code that works for both versions when the index is not `0..n`, but something else like timestamps.

For example on Mac OS X 10.7.3 with numpy 1.5.1:

``` python
Python 2.7.1 (r271:86832, Jun 25 2011, 05:09:01) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> import numpy
>>> numpy.__version__
'1.5.1'

>>> import pandas
>>> pandas.__version__
'0.7.3'

>>> a = numpy.array(range(10))
>>> s = pandas.Series(a)

>>> numpy.where(~numpy.isnan(s))
(0    0
1    1
2    2
3    3
4    4
5    5
6    6
7    7
8    8
9    9,)

>>> type(numpy.where(~numpy.isnan(s))[0])
<class 'pandas.core.series.Series'>
```

versus same system with numpy 1.6.1

``` python
Python 2.7.1 (r271:86832, Jun 25 2011, 05:09:01) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> import numpy
>>> numpy.__version__
'1.6.1'

>>> import pandas
>>> pandas.__version__
'0.7.3'

>>> a = numpy.array(range(10))
>>> s = pandas.Series(a)

>>> numpy.where(~numpy.isnan(s))
(array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]),)

>>> type(numpy.where(~numpy.isnan(s))[0])
<type 'numpy.ndarray'>
```
