i agree with jay rockfeller and steve soto, if bush really can say "with certainty" that the iranian government is intentionally supplying weapons to be used against u.s. forces, then he should show the evidence. if he can't show anyone outside his administration the evidence, then that means he's lying.
but i also think that even if bush has the goods on iran, that wouldn't justify an attack. as i said below a bombing campaign against iran would not stop the flow of weapons into iraq. if anything it would probably accelerate movement of weapons into iraq as iran gets even more incentive to bog down u.s. forces there.
i remain doubtful that the iranian government is supplying the weapons as bush is claiming. but even if i'm wrong, an attack against iran would be the worst way to handle it. if bush is building a case for an attack on iran, this is a particularly bad way of doing it. given that the u.s. simply does not have the forces to occupy iran, if anything, it's just another reason to negotiate. unlike bombing, negotiations could actually do something other than kill people.