it is impossible to prove that something is safe, you can only prove that something is harmful and thus far there isn't any credible evidence that cell phones are harmful.
If a large enough population is using X for a long time and it's mortality is not distinguishable from those not using X, then that is proof that using X doesn't kill you.
Of course, that takes decades, and can't be proved before X is widely used - but it allows us to reason about the [lack of] deadliness of various wierd cultural habits, diets, etc.
