to me, The Fountain revolved around the concept that love is eternal. it's shown thru 3 stories--1 past, 1 present, 1 future. the present is the key to understanding the movie because it involves the story that the wife is writing. that story is "the fountain" in which the two lovers are represented in both past and future. the key to jackman finding the tree of eternal life in the past is so that he and his queen could stay together, the key to jackman finding a cure in present is so he and his wife could stay together, and the the key to jackman traveling to shebulba (sp?) at the end is so that he and his wife can become the two intertwined nebulas and give birth to new galaxies (and live eternally thru them).
the whole movie is a metaphor for man's struggle to live eternally thru love. i dig the hell out of it. ^.^