Weird None /nan behaviour
Somehow I can't set single NaN entries to be None, but I can set a whole column of NaN entries to a column of None entries.  ( x[1].ix[3]=None doesnt work either)

> > > from pandas import *
> > > x = DataFrame(columns=[0,1], index=[3,4])
> > > x
> > >     0   1
> > > 3 NaN NaN
> > > 4 NaN NaN
> > > 
> > > x[0]= [None,None]   # this works fine
> > > x
> > >       0   1
> > > 3  None NaN
> > > 4  None NaN
> > > 
> > > x[1][3]=None   # this has no effect  and is my issue
> > > x
> > >       0   1
> > > 3  None NaN
> > > 4  None NaN
