Issue Using Chained Accessors with Multiple dtypes & Performance Tips
The below code should print 99.9, but instead prints 10.5.  This bug goes away if column b in the dataframe is set to all float values instead of int values.  I am running Pandas 0.12.0 and Python 2.7.3.

```
import pandas
df = pandas.DataFrame({'a':[7.3,5.1,3.0,5.5,6.4],'b':[5,4,5,1,6]})
df['a'] = df.apply(lambda x: 10.5, axis=1)
df.iloc[0]['a'] = 99.9
print df.iloc[0]['a']
```
