Thats more or less the conclusion we arrived at here - tie up the plots in the third season, finish it at the end of that, leave a few mysteries open and perhaps some room for a movie and you're *golden*. Speculation about the minutae will go on for decades and all they have to do is make whispers about a sequel series or a special final episode or a movie every couple of years and that'll keep interest high too.
Of course, they can't end lost now. No one wants to have to say they survived the great Geek riots of '06.... or took part in them... I can't imagine how they could pull the plot out to five seasons but I guess thats why I'm not writing emmy award winning drama/whateverthehelllostis. Speaking for myself, I'm liking the bizzare stuff on the assumption that it will turn out to have a reasonably rational explanation.