It hasn't happened yet, nor is there a timeline for it, but Facebook has stated that it's working on allowing its members to "organize that long list of friends into groups so you can decide more specifically who sees what." No formal announcement was made, but you can see the little tidbit--along with something about forthcoming "daily digest" e-mail options--on the What's New on Facebook page. It looks like TechCrunch was the first to spot this.
"Friend grouping" is a move that, unless the company really screws it up, Facebook members are very likely to applaud.
Technically speaking, it's hardly revolutionary. The Six Apart-owned blogging pioneer LiveJournal, for example, has allowed the simple creation of "custom friend groups" for years. But what you have to understand about Facebook is that millions of young "early adopters" signed up with the (not particularly forward-thinking) expectation that the only people who'd be seeing their profiles were their college friends and other contemporaries.
Several years later, many of those then-college students are now young professionals; Facebook's new open-registration policy has allowed their colleagues and relatives to create accounts; and there are a whole lot of St. Patrick's Day Keg Race photos circa 2004 (and office happy-hour photos circa 2007) that perhaps those members don't want their bosses or parents to be seeing.
Presumably, if Facebook members are able to sort their contacts into custom groups, they'll be able to display varying degrees of profile data to those groups. That way, for example, your boss might only be able to see your profile photograph and work information, whereas your college buddies would be able to keep reminiscing about the old days with those St. Patrick's Day snapshots.
This will put Facebook even further ahead of "openness"-advocating rivals like Plaxo Pulse, which lets members group their contacts into "friends," "family" and "work" but doesn't allow any custom functionality.