What language are you hoping to get them started on? Python? Scheme? I have been wanting to start with Scheme... but that is because I read PG's essay and got bitten by the bug.
Keeping the lecture time down to 45 minutes sounds wise. Students are already swamped with class hours so keeping the weekend time burden down is probably the right move.
Perhaps you could set up teams that could compete with each other through a series of programming challenges? You could find a local business who would sponsor a prize, or a restaurant that would offer a dinner for the winning team.
Be sure to post updates on your group's website. Other people, like myself, are wanting to do the same thing at other schools and would be encouraged to hear about your progress. Your website could also link to the students' work on Github once they are ready to step into their first real projects.
Your program sounds exciting! Keep us up to date.