Unexpected behaviour when converting datetime column to np.datetime64
To me this behaviour appeared to be weird.
Numpy 1.7 and latest Pandas

```
from pandas import *
from datetime import datetime
x = DataFrame([datetime.now(), datetime.now()])
print x[0].dtype
print to_datetime(x[0]).dtype
x[0] = to_datetime(x[0])
print x[0].dtype, "shouldn't this be datetime64?"
x[1] = to_datetime(x[0])
print x[1].dtype, "this is"

object
datetime64[ns]
object shouldn't this be datetime64?
datetime64[ns] this is
```
