Updating DataFrame element silently fails
I have a DataFrame that I am using to keep track of student grades during a course. Here is a sample record:

```
In [18]: grades.ix[8]
Out[18]: 
Unnamed: 0               8
Name          Johnny Student
hw1                     50
hw2                     50
hw3                     50
hw4                     50
exam                   100
grade                  100
Name: 8
```

Let's say I want to update Johnny's exam score to 95:

```
In [19]: grades.ix[8]["exam"]
Out[19]: 100

In [20]: grades.ix[8]["exam"] = 95

In [21]: grades.ix[8]["exam"]
Out[21]: 100
```

Its not clear why this should fail, but more importantly, it should not fail silently.
