Warning/Exception when querying dtype with use_inf_as_null
I see `Exception TypeError: 'data type not understood' in 'util._checknull_old' ignored` in the ipython notebook warning color after turning on `use_inf_as_null`.
![screen shot 2014-11-03 at 10 27 29 am](https://cloud.githubusercontent.com/assets/379413/4884670/fbc6a18a-636d-11e4-950e-6c18f66480f8.png)

The code to reproduce it is 

```
import pandas as pd
pd.options.mode.use_inf_as_null = True
pd.DataFrame([[1]]).dtypes
```
