The two changes take effect Jan 28, 2013:
1) Instead of using just a first name, last initial (e.g. Adena S.) to indicate check-ins publicly, on that date, full names will be used (Adena Schutzberg). Foursquare says the old way is confusing, but explains any user can change their full name if they choose, perhaps to just first name, last initial.
2) Until the 28th venues participating in Foursquare could see check-ins from just the previous three hours. That's not too useful if someone didn't look up and act on that information regularly. Now, venues can see it for longer. How much longer? Unclear.
The company also issued a "Privacy 101" document to explain ist vision of user privacy. Interestingly, it was last updated Jan 28, 2013.