Well, I guess this depends on how you do your math, but if you count each hour-long installment of Lost as an episode, 119 will be the final episode of Lost when it goes off the air in May 2010, after six seasons.
To the best of my knowledge, the decision to end a show three years in advance, regardless of its ratings, is unprecedented in network history. Sci-fi saga Babylon 5 was theoretically structured for a 5-year narrative arc, a plan which went to hell near the end of the fourth season when the remaining plot points were wrapped up in anticipation of the show's cancellation... leaving the show in need of a new plot when it returned for a fifth season after all. Of course, ABC's announcement doesn't indicate what would happen if the show were to tank, ratings-wise, before the anticipated end-date -- but since Lost, even at its worst moments, has never dropped far below the Top 25 shows on television, it seems like a reasonable bet that the show will make it until the end of its run.
As one friend pointed out to me this morning, Lost will not be the first show to leave television while it still has a strong audience; when Seinfeld wrapped up, the series was doing well enough to have guaranteed it at least another season. The difference, of course, is that as one of the pioneers in television's current wave of complex serialized drams, Lost is attempting a structural feat that is almost impossible under the normal confines of network television.
(The prolific Sam Ford has discussed the challenges and difficulties that serial narratives face on network television in several past entries: see here, here, and here for a more detailed discussion of the topic. )
Lost's co-EPs, Damon Lindelof and Carlton Cuse, announced several months ago that they had approached ABC about setting an end-date for the show, an unorthodox request that would allow them to plan a specific timeline for addressing the various mysteries and puzzles that lie at the heart of the show. In particular, a firm end-date would allow Lost to address the rising concern among viewers -- common to all heavily serialized mysteries -- that the show was "making things up" as it went along, and posing questions for which it had no answers.
According to this morning's Variety, however, this was not a casual request: in their recent contract negotiations, Lindelof and Cuse demanded an end date as one of their unnegotiable terms.
It will be interesting to see whether this decision results in a noticeable upturn in the show's ratings, as exasperated viewers return to the fold, or a further decline, as more fans opt to wait until the show's 2010 conclusion to decide whether to invest another 48 hours of their time on DVD.