According to Zap2It, the finale will offer a lot of closure, particularly for main characters Peter Bishop (Joshua Jackson), Olivia Dunham (Anna Torv) and Walter Bishop (John Noble) -- enough, one assumes, to serve as a satisfactory series finale if the show's not renewed. But, according to Lance Reddick, who plays Homeland Security agent Phillip Broyles, there's a crazy cliffhanger that's also been shot that could fuel a whole new season, presuming that show's given one:
"Something happens at the very, very end of the season finale," says Reddick. "It may or may not stay; I don't know. It could open up an entire new season." If FOX does cancel the series (or put it on hiatus forever), then those additional scenes may hit the cutting room floor so that the season finale will serve well as a series finale. "It could be a cliffhanger, but the thing that's beautiful about the way that the season ends is that it seems to answer almost all the questions that started in Season 1... except for that one story," Reddick says.
While I'm sure everyone behind the show wants one more season, it's a bit of a luxury to be able to plan ahead for a possible ending. If the series doesn't come back, at least that cliffhanger will make for one hell of a DVD extra.