I would like to point out that legends of Excalibur (originally Caledfwlch) and myths of Greek heroes receiving magic swords from the gods are just that, myths and legends, often written well after the supposed fact. The difference i see with keris and it's concepts of tuah and isi as well of concepts of royal pusaka and the abilities to disperse power and rights of rule through the passing of a specific blade to subservient leaders throughout one's kingdom was not myth or legend, but a living cultural reality. Whether or not one believes that these powers resided within the keris or not is irrelavent. That the people of that culture believed was all that was need to make it true for them. These practices and beliefs are well recorded and documented and can't really be compared with magic swords of the gods or heroes of fiction.
BTW, it seems that Excalibur and the Sword in the Stone were derived from separate myths that at some point became one.
http://en.wikipedia.org/wiki/Excalibur