Attribute access bug?
Hello,

I'm using Python 7.2.3 and Pandas 0.12.0.
When I run the code below, it works as expected, by creating new items and updating them.

```
quotes_premiums = quotes_cc_returns.transpose(2, 1, 0)

quotes_premiums['RiskPremium'] = quotes_premiums.CC_Returns.sub(ff_data_factors_subset.RF, axis=0)

quotes_premiums['MktRiskPremium'] = 0.0
quotes_premiums['MktRiskPremium'] = quotes_premiums.MktRiskPremium.add(ff_data_factors_subset.MktMinusRF, axis=0)

quotes_premiums.MktRiskPremium.iloc[0] = None

quotes_premiums = quotes_premiums.transpose(2, 1, 0)

quotes_premiums['VFINX'].head()
          Open    High     Low   Close  Volume  AdjClose  CC_Returns  RiskPremium  MktRiskPremium
Date                                                                                                 
2001-01-31  118.44  126.88  118.44  126.18       0     99.74         NaN          NaN             NaN
2001-02-28  126.88  126.88  114.65  114.65       0     90.62   -0.095892    -0.099792         -0.1008
2001-03-31  114.77  116.99  103.12  107.07       0     84.86   -0.065672    -0.070072         -0.0727
2001-04-30  105.74  115.75  101.83  115.39       0     91.45    0.074790     0.070890          0.0795
2001-05-31  116.95  121.36  115.11  116.14       0     92.05    0.006540     0.003340          0.0073
```

However, if I make a single change, using what I think is called Attribute access, it stops updating the MktRiskPremium item (column after transpose).

```
quotes_premiums.MktRiskPremium = quotes_premiums.MktRiskPremium.add(ff_data_factors_subset.MktMinusRF, axis=0)

quotes_premiums['VFINX'].head()
          Open    High     Low   Close  Volume  AdjClose  CC_Returns  RiskPremium  MktRiskPremium
Date                                                                                                 
2001-01-31  118.44  126.88  118.44  126.18       0     99.74         NaN          NaN               0
2001-02-28  126.88  126.88  114.65  114.65       0     90.62   -0.095892    -0.099792               0
2001-03-31  114.77  116.99  103.12  107.07       0     84.86   -0.065672    -0.070072               0
2001-04-30  105.74  115.75  101.83  115.39       0     91.45    0.074790     0.070890               0
2001-05-31  116.95  121.36  115.11  116.14       0     92.05    0.006540     0.003340               0
```

Is this normal?

Thanks,

JM
