Spoilers much?
On her way back to where the Oceanic Six sat in the cargo area of the plane, Ms. Decker acknowledged someone sitting close to the cockpit (Ep: 4x12, There's No Place Like Home Part 1). Was this a crew member, or someone from the island that we already know?
At first I thought maybe it was Ben, but Sayid was surprised that Ben was off the island, so it couldn't have been him. I think it will be revealed later in the series. The reasons I don't think it's a crew member are:
Ms. Decker said hi to him and it seems she would have already exchanged pleasantries with the crew. The producers had her acknowledge this person, but then didn't show this person. Many people missed that she'd even acknowledged anyone, but after the big reveal, if viewers go back and watch this scene, then they would likely catch it.