int64 Series and NA assignment
I think a bug:

In [1]: s = Series(arange(10))

In [2]: s
Out[2]: 
0    0
1    1
2    2
3    3
4    4
5    5
6    6
7    7
8    8
9    9

In [3]: s[::2] = np.nan

In [4]: s
Out[4]: 
0   -9223372036854775808
1                      1
2   -9223372036854775808
3                      3
4   -9223372036854775808
5                      5
6   -9223372036854775808
7                      7
8   -9223372036854775808
9                      9

In [5]: s.dtype
Out[5]: dtype('int64')

Dataframe seems fine:

In [12]: df = DataFrame(np.random.random_integers(5, size=(5,5)))

In [13]: df
Out[13]: 
   0  1  2  3  4
0  5  5  3  1  2
1  1  3  3  3  2
2  1  5  2  2  3
3  5  3  1  1  4
4  2  5  2  3  2

In [14]: df[2] = np.nan 

In [15]: df
Out[15]: 
   0  1   2  3  4
0  5  5 NaN  1  2
1  1  3 NaN  3  2
2  1  5 NaN  2  3
3  5  3 NaN  1  4
4  2  5 NaN  3  2
