I would say begin with 2.6 since the vast, vast majority of documentation regarding Python will be applicable to 2.6 as well most open source projects you may want to contribute to will be in 2.6 for awhile. Then, once you have a good foundation in 2.6, you can learn 3.0. That way you can kind of appreciate how the language has evolved and where the "aesthetic" of the code comes from.
