.ix[] inconsistently assigns right hand side
This is with pandas 0.12.0.dev-be25266:

In [1]: import pandas as pd

In [2]: df = pd.DataFrame({'a':[1,2,3],'b':[0,1,2]})

In [3]: df.ix[[0,2,],'b'] = [100,-100] # Works as intended

In [4]: print df
   a    b
0  1  100
1  2    1
2  3 -100

In [6]: df = pd.DataFrame({'a':range(4)})

In [7]: df['b']=pd.np.nan

In [8]: df.ix[[1,3],'b'] = [100,-100] # Both rows get assigned 100

In [9]: print df
   a    b
0  0  NaN
1  1  100
2  2  NaN
3  3  100

In [10]: df['b'].ix[[1,3]] = [100,-100] # Works as intended

In [11]: print df
   a    b
0  0  NaN
1  1  100
2  2  NaN
3  3 -100
