Anyone else read or heard that the producers may actually finish the series with a feature length film? Similar to Forefox/Serenity idea i suppose, Joss Wheadon has started something.
Llamatron's right about Firefox/Serenity....
As for Lost there are so many rumours bandied around about Lost its impossible to predict what will happen in 5 years time. There are 7 series planned and the writters know how it will end but i doubt they will choose an approach that has been done before.