unanticipated get_dummies behavior with missing values
s = pd.Series(['A', 'B',  np.nan, 'C'])
print s

0      A
1      B
2    NaN
3      C

print pd.get_dummies(s)
   A  B  C
0  1  0  0
1  0  1  0
2  0  0  1
3  0  0  1

In the third row,col C, the indicator should be 0 not 1.
Seems you'd either want to add a column for NaN values or fix the issue above

sorry for bad formatting...
