How will the dynamics of Facebook change after they introduce Check-ins?

The dynamics of Facebook may or may not change at all.  Because of the sheer number of Facebook (500 Million) users, any new feature added will likely take a while for significant user adoption.  An overnight change (as one might expect after a feature is introduced) is hard to imagine.

Whether or not users broadly adopt announcing their location is highly dependent on the user.  Most people would prefer not to broadcast their location to 150 people (average number of friends).  Facebook has gotten large enough where its not just your close friends and family.  Your friends may include elementary school friends whom you haven’t seen in a decade (effectively strangers), coworkers that you haven’t worked with in 5 years, someone that lived in the same hall as you in college.  The term friend is hardly indicative of the connections fostered.

Broadcasting location to such a group will likely make users (especially women) uncomfortable.  But with a pool of 500 Million, there will likely be those that check in everywhere, others who check in for incentives, and still others who chose not to check in at all.