Okay this code snippet will help you print the last modified entities. Give it a shot.
>>> import boto
>>> s3 = boto.connect_s3()
>>> bucket = s3.lookup('mybucket')
>>> for key in bucket:
print key.name, key.size, key.last_modified
index.html 13738 2012-03-13T03:54:07.000Z
markdown.css 5991 2012-03-06T18:32:43.000Z
>>>
This may not a be a perfect solution because last modified is a string. You may also consider using boto.utils.parse_ts to parse a string