Hi:
You can cut off a piece weigh it and then dry it for awhile in an oven at a low temp and weigh it again to really see how much moisture is in it. When it stops loosing weight its dry. I do furniture building and my suggestion is that you once you plane it it get it locked into the steel bed frame. This will help with twisting and warping. You could even leave it thick and just clamp it in place for awhile.
Check out this site. It has lots of info on bed construction:
http://mar-k.com/installation_hints.html