Exception using float as 'diff' periods
from pandas import DataFrame
df = DataFrame({'US': [1, 2]}, index=[datetime(2012,1,1), datetime(2012,1,2)])
df.diff(1.)

This succeeded in 0.9.0. Now it raises:

pandas\core\frame.pyc in diff(self, periods)
   3937         diffed : DataFrame
   3938         """
-> 3939         new_blocks = [b.diff(periods) for b in self._data.blocks]
   3940         new_data = BlockManager(new_blocks, [self.columns, self.index])
   3941         return self._constructor(new_data)

pandas\core\internals.pyc in diff(self, n)
    292
    293     def diff(self, n):
--> 294         new_values = com.diff(self.values, n, axis=1)
    295         return make_block(new_values, self.items, self.ref_items)
    296

pandas\core\common.pyc in diff(arr, n, axis)
    470     if arr.ndim == 2 and arr.dtype.name in _diff_special:
    471         f = _diff_special[arr.dtype.name]
--> 472         f(arr, out_arr, n, axis)
    473     else:
    474         res_indexer = [slice(None)] \* arr.ndim

pandas\algos.pyd in pandas.algos.diff_2d_int64 (pandas\algos.c:14030)()

TypeError: 'float' object cannot be interpreted as an index

This might very well be a desired change - but it's something we'll have to work around. I'm just reporting it for completeness :-)
