Minor tweaks.

I like the direction you've taken in giving a full example that they'll have to use TDD on from scratch. 

I think TDD is very simple when making a minor tweak on a large project - but a more full understanding of the small iteration cycle, along with the ability to break a problem down into small steps, needs to be grasped when starting something from scratch. Hopefully the students will attain that through the tic-tac-toe :)

One suggestion: Perhaps a section on why we need to test at all, why we bother and what we're going to gain from it before Part I?