Since your list is already in ascending order, we can simply reverse the list.
>>> timestamps.reverse()
>>> timestamps
['2010-04-20 10:25:38',
'2010-04-20 10:12:13',
'2010-04-20 10:12:13',
'2010-04-20 10:11:50',
'2010-04-20 10:10:58',
'2010-04-20 10:10:37',
'2010-04-20 10:09:46',
'2010-04-20 10:08:22',
'2010-04-20 10:08:22',
'2010-04-20 10:07:52',
'2010-04-20 10:07:38',
'2010-04-20 10:07:30']