Python 2.5 is a good baseline, since that's what a lot of distributors are shipping.

Besides, the syntactic additions made to the language in Python 2.6 are almost all meant to provide a first step toward Python 3.0, or make standard things that were available as future imports in 2.5. So you're not really missing anything there.

I remember a post that CPython 2.6 was still in development when they started Jython 2.5. They were making the leap from 2.1 or 2.2 so they didnt want to add an undefined release to the pot.

It's a lot of work, and 2.5 is still the most used. I think the plans are to go straight to 3.0 from here, rather than release a 2.6 version.