Caveat: I just started thinking about this, so I don't feel particularly articulate.
After reading Roy's post of Kent Beck's discussion “Invest in the design of the system every day”, I realized that's what I do for project planning. Every day, I'll adapt the work I've planned to do, to meet the needs of the project. That doesn't mean I'll necessarily change direction, but I'll think about the current project state, and what else we need to do.
We've all see the phenomenon that as soon as you've scheduled the project, the schedule is out of date. If you plan to invest in replanning and rescheduling, that doesn't matter.
This works for programs, product management, all the work that's affiliated with product development, but may not be strictly considered product development. Adapting your work to the current state is investing in the design of your work every day.