If you recall, this season of Lost was originally cut short by 3 episodes due to the strike. Then, the producers decided to squeeze in one extra episode this season (tacking it on to what is sure to be an explosive 3 part finale).
More good news: Each of the remaining seasons of Lost was originally supposed to have 16 episodes. Now, the final 2 seasons will each have 17 episodes each to make up for the final 2 episodes that were cut from the current season.
Source: TV Guide