Datetime index error?
I have a dataframe of tweets, where the "created_at" column is stored as a datetime object. I just upgraded to the latest version of pandas, and now get this error:

```
df['created_at'].map(lambda d: d.strftime("%Y%m%d"))

TypeError: Argument 'arr' has incorrect type (expected numpy.ndarray, got DatetimeIndex)
```

strangely enough, everything works correctly if I do this:

```
df2 = df.set_index('created_at')
df2.index.map(lambda x: x.strftime("%Y%m%d"))
```

and this:

```
def f(d):
    return d['created_at'].strftime("%Y%m%d")

df.apply(lambda row: f(row), axis=1)
```

What is going on here?
