Start with 2.x. Most existing libraries will be on 2.x for a long time. Last year, Guido himself said that it would be "two years" until you needed to learn 3.0; there's still another year left. Personally, I think it will be longer. People writing code on 2.x can learn how to use the 2to3
tool and have code that works on both versions. There is no 3to2, so code written for python 3 is significantly less valuable.
Thats not to mention how disappointing it will be for your students to learn that python 3 is not installed on their Linux computer ("/usr/bin/python
" will be python 2.x for the next 5 years, at least), that there is no django for python 3, no wxwindows for python 3, no GTK for python 3, no Twisted for python 3, no PIL for python 3... the real strength of Python has always been in its extensive collection of libraries, and there are very few libraries for python 3 right now.
If your tutorial is well written, you should easily be able to update it to python 2.6, 2.7, and eventually python 3.