<rant>
You should deffinetly go with Python 2.6.x.
Python 3.x still lacks on most of the usefull libraries that make Python useful. Python 2.6.x includes many of the features included in 3.x, and the whole 2.x codeline will be supported for years. Python 2.7 is planned, and wouldn't be surprised if there were a 2.8 some time in the future.
You can write using some of the most useful features from 3.x in 2.5, using the from __future__ import
s.
Remember 2.x isn't going anywhere in the short term. Many distributions still ship with Python 2.4, and there are enough people out there mainaining compatibility with 2.3 (Django, for example).
The differences between 3 and 2.6 are small enough be easy to automatically port 2.6 code to 3 (with 2to3) when you really want to, while still having access to your needed libraries in the meantime.
</rant>