Actually the Casimir effect just reduces the "ambient" pressure, it doesn't create negative pressure. http://en.wikipedia.org/wiki/Casimir_effect

Edit: Whoops, looks like it might work anyway http://en.wikipedia.org/wiki/Casimir_effect#Wormholes

Edit2: Actually I think that contradicts this http://en.wikipedia.org/wiki/Wormhole#Raychaudhuri.27s_theor... which makes more sense to me. Unless you have already curved spacetime, a Casimir effect is not enough to stabilize a wormhole.