Timestamp indexing bug
```
from datetime import datetime
import numpy as np
from pandas import DataFrame, Timestamp, DatetimeIndex
import pandas.core.datetools as datetools

columns = DatetimeIndex(start='1/1/2012', end='2/1/2012', freq=datetools.bday)
index = range(10)
data = DataFrame(columns=columns, index=index)
t = datetime(2012, 11, 1)
ts = Timestamp(t)



This raises an exception:

data[ts] = np.nan

while this works:

data[t] = np.nan
```
