I (now) know I'm supposed to say "setup.py install" but it was a bit unfriendly.
(this time, I'm on RHEL6, with python 2.6.6 - no reason to believe it's different under python 2.4 though)
[root@cnsutil0 s3cmd-1.5.0-rc1]# python setup.py
Using xml.etree.ElementTree for XML processing
Traceback (most recent call last):
File "setup.py", line 38, in <module>
if sys.argv == "sdist":
IndexError: list index out of range