clearly by the accounts in this thread, and also by my experience, it's rather touch-and-go with keeping shrimp and bettas in the same tank.
My betta lived peacefully with 20 ghost shrimp in his tank for over 2 months.. then, they all disappeared in a matter of days.. i looked closer, and noticed he had a couple of legs sticking out of his mouth. lol.
if your shrimp mean the world to you, i wouldn't risk it.. but if they're 'expendable' members of your cleanup crew, it's worth a shot. you'll never know until you try.