According to the New York Times, Facebook is going to roll out full-blown location sharing in April. From the NYT's Bits Blog:
Facebook Will Allow Users to Share Location - Bits Blog - NYTimes.com
At that time, the company also offered some foreshadowing of the new feature: “If we offer a service that supports this type of location sharing we will present you with an opt-in choice of whether you want to participate.”
Facebook has been working on a location-based tool for close to a year, but decided to wait until the product was completely ready for mainstream adoption before announcing it, said the people with knowledge of the project.