According to this article, ABC plans to set and announce an endpoint for Lost. I'm all for it. The show really needs one if only so fans believe there actually is some plan. It would even be beneficial to ABC (if disingenuous) for them to announce an endpoint even if it didn't exist.
But I'm skeptical that they'll announce one. TV networks tend to milk their golden geese for all they're worth; most shows end up drawn out so far that they jump the shark.