It looks like it'll hold fine at the bottom but unless you use those little blocks all the way up I think the tops of the outer boards might warp away from each other due to the absorption of water, drying out, absorption, etc. And depending on which way the grain is running that will come into play also. (Hank will know more about this than me...Oh Vic!!)
All in all I think you're gonna have some great raised beds! I'm already thinking of making some like your diagram!
