NaT Not Correctly showing up as null value
On pandas 0.12:

```
pd.DataFrame( pd.Series([datetime(2012,12,12).replace(tzinfo=pytz.utc)] + [pd.NaT]*100))
```

I would argue this should show 1 non null-value:

```
<class 'pandas.core.frame.DataFrame'>
Int64Index: 101 entries, 0 to 100
Data columns (total 1 columns):
0    101  non-null values
dtypes: object(1)
```
