In the course of testing some changes against Python 2.4, I was remembering this conversation. I looked up RHEL Python versions [1] and end-of-life phases [2].
Which end-of-life phase do you want duplicity to target?
If it's the normal 7-year phase, that means we should support 2.4 through early 2014 (and actually, that we should still be supporting 2.3).
There are other phases of end-of-life (4 and 5 years each) you could target if you wanted to be more aggressive and a longer phase (10 years) if you wanted to be more conservative.
[1]
http://distrowatch.com/table.php?distribution=redhat[2]
https://access.redhat.com/support/policy/updates/errata/
-mt