You can use a lot Python 3 features in 2.6 and later. This document, http://docs.python.org/3.0/whatsnew/3.0.html describes the difference between the two. So while I agree with the other posters that it it would be better to start with 2.6 I also think you could minimize your upgrade pain by knowing the differences and doing as many things as you can in the Python 3 style.
