creating Panel from dict of DataFrames loses dtypes
in the example below, "a" is a mixed type dataframe. Once included in the Panel, all columns become of type object:

In [249]: a=DataFrame(randn(30,3),columns=('a','b','c'))

In [250]: b=DataFrame(randn(30,3),columns=('a','b','c'))

In [251]: a['b']=a['b'].astype(object)

In [252]: a.dtypes
Out[252]: 
a    float64
b    object
c    float64

In [253]: b.dtypes
Out[253]: 
a    float64
b    float64
c    float64

In [254]: Panel({'1':a,'2':b})['1'].dtypes
Out[254]: 
a    object
b    object
c    object

In [255]: Panel({'1':a,'2':b})['2'].dtypes
Out[255]: 
a    float64
b    float64
c    float64
