The Tail Section

Episode 2.21: ?
Turns out that supercool black-light reveal is a major plot point that carries on through the rest of the season. “Oh yeah, big time,” Damon said. “I mean, it plays huge in the finale, and even in episodes 21 through 24. Twenty-one is an episode that is very much on the sort of fundamental axis of that map, and it’s a Locke and Eko story, which is going to be awesome. [Executive Producer] Carlton [Cuse] and I wrote it. That episode is just called ‘?’ because that is the symbol Locke remembers from the map.”

General
We can also expect a little (or a lot of) romance between 2 of the losties…

Source: Kristin @ E!Online
April 1st Special Edition