it would used to be my left arm, but while i was recovering from a very bad strain in my left arm this past spring and through the summer it's been my right arm that would be on top. i sleep on my side and frequently scrinch my arms up close to me while sleeping, so i know.
so, now i think it's a toss up, a 50/50 chance as to which of my arms would be on top. though my instinct still seems to be to protect my left arm.
interesting, if there might be a correlation between if you're right or left handed. i'm right handed.