Edit: A little coverage in VentureBeat http://venturebeat.com/2012/04/19/health-tracking-app-needto... I just had to check. :)
This model (free service/soft + hardware) reminds me of sports-tracker.com  (one of few sites I simply love). They service (app, site) is also free and they sell heart rate monitors.
With same model (do free service, sell hardware), there is one difference which is interesting. Sports-Tracker sells their own hardware, MyFitnessPal just resells Fitbit.
Added: I just hope they (MyFitnessPal) do not sell their databases.