I'd say start with the 2.x series. They are still being released (and features are backported from 3.x), and they remain compatible with previous releases, so all the bindings, etc.. keep working.

"Mainstream" Python 3.x is still not really the case, but the good news is that it's more of a "clean-up" than a re-invention, so the only differences that you will note between 2.x and 3.x (when it's time to move to the newer version) is that it's just a clean-up of the language and the libraries (so 3.x will feel more "logical" and "tidy" than 2.x). But there are lots of docs on the web describing the difference between 2.x and 3.x.

For Maemo development, just stick to Python 2.5 and then use whatever Python version will be used in future versions.

HTH.