possible error in mean of dataframe with nan values?
I am not sure if I am doing something wrong, but this looks like a bug to me:

In [116]: ra.ix[:5,:5]
Out[116]: 
           005665            023740 028758 040828            045449  
2011-05-09 NaN               NaN    NaN    NaN               NaN  
2011-05-10 -0.00284467268624 0.0    0.0    -0.002844627571   0.000353248710642
2011-05-11 -0.00674436854779 0.0    0.0    -0.00674432257973 0.0  
2011-05-12 0.0117105570627   0.0    0.0    0.0117104073878   0.0  
2011-05-13 0.00367090573068  0.0    0.0    -0.00102855609881 0.0              

In [117]: ra.ix[:5,:5].mean(axis=0,skipna=True)
Out[117]: 
005665    0.0014481
023740    0.0000000
028758    0.0000000
040828    0.0002732
045449    8.831e-05

In [118]: ra.ix[:5,:5].mean(axis=1,skipna=True)
Out[118]: 
2011-05-09    NaN
2011-05-10    NaN
2011-05-11    NaN
2011-05-12    NaN
2011-05-13    NaN

In [122]: ra.ix[1:5,:5].mean(axis=1,skipna=True)
Out[122]: 
2011-05-10   -0.0010672
2011-05-11   -0.0026977
2011-05-12    0.0046842
2011-05-13    0.0005285

In step 118, I would expect the same output I get at step 122. Maybe the check if values are all NaN on a given row are done on the wrong axis?
