  We improve the study of the lack of perturbative unitarity of noncommutative
space-time quantum field theories derived from open string theory in electric
backgrounds, enforcing the universality of the mechanism by which a tachyonic
branch cut appears when the Seiberg-Witten limit freezes the string in an
unstable vacuum. The main example is realized in the context of the on-shell
four-tachyon amplitude of the bosonic string, and the dependence of the
phenomenon on the brane-worldvolume dimension is analysed. We discuss the
possibility of a proof in superstring theory, and finally mention the NCOS
limit in this framework.
