I think both positive and negative reinforcement are effective. It obviously depends on the person, situation and other factors. My thought with this idea is that the positive re-inforcement aspect of activity when it comes to activity monitors is already being tackled by the big companies, fitbit, garmin, apple health, google fit, etc. However on the 'penance' side of things, it's underrepresented and the options are far from streamlined and easy imho.
My opinion on Beeminder is that it's designed for highly-technical people. The idea would be to present a solution that is similar to Beeminder that anybody can easily understand and use. There are also other highly motivating aspects that beeminder doesn't use such as social accountability (fitbit), charities (StickK and many others), and competition (fitbit, apple, etc.).
Thanks again for the thoughts and inquiry, I'd love to keep the convo going. My current goal is to flesh out any lingering thoughts on how the idea could be improved so that I can create more of a polished model to present to the public and gage interest.