Thanks for letting us know the details on how the new system works. Unlike most I actually like this new activity system a lot. Its a nice change, and reputation can be less post based and more time spent reading the forums. Goodjob!!
My theory about activity is that the quality of your posts, as well as the number of replies you give to the same topic, and the time spent on the forum determines it. If you reply more to a thread with quality posts, in theory, you are more "active."
How would it be able to check the quality of your posts? I kinda like the idea tho.