DataFrame.applymap with identity function changes dtype
from datetime import datetime
from pandas import DataFrame
df = DataFrame({'x1': [datetime(1996,1,1)]})
df2 = df.applymap(lambda x: x)

Issue:
type(df['x1'][0]) == datetime.datetime
type(df2['x1'][0]) == pandas.lib.Timestamp

This repros in the 11/27/2012 cut of 0.10.0 - but not in 0.9.0 release.
