>It's also just much more fun to work with people who love programming, and an easy way to tell (even if it's susceptible to a few false negatives like your friend) is to find out if they program in their own time.

The thing is though, you can love programming and find that 40 hours a week of it is more than enough. I think what you'd find with this heuristic is that it tends to favor young people. It's a lot easier to be enthusiastic about some side project when you're 20 than when you're 40 and have a lot of responsibilities.