Well..."extra" is kind of overstating things, since we're still getting less than the 16 Lost episodes that we were originally promised (before the writer's strike), but it has now been confirmed that Lost's season finale will be 3 hours long!!
It will take place over the course of 2 nights, bringing to total number of episodes in this season back to 14 (instead of the 13 that we thought we were going to be getting, which is why it is "extra").
The first hour of the finale will air on May 15 and the second two hours will air on May 29th.
14 episodes of Lost is better than 13, so everybody rejoice!