Which Version of Python to Use for Maximum CompatibilityThis my Answer to the stackoverflow question: Which Version of Python to Use for Maximum Compatibility:
As python is in kind of an transition phase towards python 3 with breaking backward compatibility I don't think it is a good idea to go python 3 only. Based on the time line there will be at least one or two following releases of the 2.x series after 2.6/3.0 in october.
Beside not having python 3 available on your target platforms, it will take some time until important external python libraries will be ported and usable on python 3.
So as Matthew suggests staying at 2.4/2.5 and keeping the transition plan to python 3 in mind is a solid choice.