Well, Paul Armentano at NORML reports  that a new study  has proven that the whole thing was just a bunch of crazy talk:
“[T]he expected rise in diagnoses of schizophrenia and psychoses did not occur over a 10 year period. This study does not therefore support the specific causal link between cannabis use and incidence of psychotic disorders. … This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence.”
In non-sciency terms, this means that when rates of marijuana use go up, rates of mental illness do not. Therefore, we can conclude that marijuana apparently doesn't cause anyone to develop psychosis who otherwise wouldn’t have.
It's really a shame that this now-debunked junk science about marijuana and psychosis led the British government to increase penalties for marijuana . But, as we know all too well, fits of ignorance and distortion are causally linked to an increased risk of bad drug legislation.