fillna() on Series or DataFrame containing datetime64 mess the values
s = pd.Series([pd.NaT, pd.NaT, '2013-08-05 15:30:00.**000001**'])
print s
0                          NaT
1                          NaT
2   2013-08-05 15:30:00.**000001**
dtype: datetime64[ns]

print s.fillna(method='backfill')
0   2013-08-05 15:30:00.**000001024**
1   2013-08-05 15:30:00.**000001024**
2   2013-08-05 15:30:00.**000001024**
dtype: datetime64[ns]

This happens with pandas 0.13.1 and numpy 1.8.0
