A very interesting article making a scholarly case for the antiquity of the canon; How Old is the Suttapitaka by Alexander Wynn. He makes the argument that the Pali Canon was closed to new material at a very early date. One of his key arguments is that stories and doctrines that are found in other recensions are relegated to the Pali commentaries. This would indicate that the Theravadin sangha in Sri Lanka were unwilling to revise the canon, even though they were keeping abreast of developments in India.
Thanks Eisel for pointing this out to me.