Confusing pd.to_datetime behavior
The docstring says
## Parameters

arg : string, datetime, array of strings (with possible NAs)

Without any date format option.

I do get some weird output though:

```
In [4]:

pd.to_datetime('121001')
Out[4]:
datetime.datetime(2012, 10, 1, 0, 0)

In [5]:

pd.to_datetime('121001', dayfirst=True)
Out[5]:
datetime.datetime(2012, 10, 1, 0, 0)

In [6]:

pd.to_datetime(['121001'])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-e09fc721e662> in <module>()
----> 1 pd.to_datetime(['121001'])

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/tseries/tools.pyc in to_datetime(arg, errors, dayfirst, utc, box)
    106 
    107         try:
--> 108             return _convert_f(arg)
    109         except ValueError:
    110             raise

/usr/local/lib/python2.7/dist-packages/pandas-0.9.0rc2-py2.7-linux-x86_64.egg/pandas/tseries/tools.pyc in _convert_f(arg)
     80                 return DatetimeIndex._simple_new(values, None, tz=tz)
     81             except (ValueError, TypeError):
---> 82                 raise e
     83 
     84     if arg is None:

ValueError: Out of bounds nanosecond timestamp: 121001-01-01 00:00:00
```
