To become mayor of a place, you need to have checked in more days than anyone else over the last two months (60 days), so only one check-in per day counts. Oh, you also need to have uploaded a profile photo from your settings page (no one wants a faceless dictatorship!). And yes, we also made changes that prevent registered "Managers" and "Employees" from becoming mayors of their own venues.
Facebook Places the oncoming steamroller Foursquare Techmeme