This is now available in version 0.5.3 (I just pushed this).
>>> from ggplot import *
>>> import numpy as np
>>> import pandas as pd
>>> df = pd.DataFrame({ "x": np.arange(0, 10), "y": np.arange(0, 1, 0.1) })
>>> ggplot(df, aes(x='x', y='y')) +\
... geom_point() +\
... scale_y_continuous(labels='percent')