The latest effort in green burials is Capsula Mundi. The people behind it (who are based in Italy) write:
Capsula Mundi is a container with an old perfect shape, just like an egg, made with modern material -starch plastic- in which the dead body is put in a fetal position. Capsula Mundi is planted like a seed in the soil, and a tree is planted on top of it. The tree is chosen when the person is alive, relatives and friends look after it when death occurs. A cemetery will no longer be full of tombstones and will become a sacred forest.
Seems like a nice idea, though they say it's currently illegal in Italy, and I'm sure it's probably illegal here in the States as well.