Ask HN: Can anyone explain how the new scoring system on Foursquare works?

I am a new user of foursquare and one thing I absolutely love about the new scoring system is how it changes user points based on the interaction with the system.

Does anyone know how to design such a system. I kinda figured out it should be a function of checkin time and points scored like score = (total_points/((time_last_checkin)^some_constant).

Does anyone has a better explanation or a better score function?