Agnostics, sure; they often seem the most rational and sane as a group, but positive atheism just takes the same leap of faith rt referenced; it simply leaps in the other direction. Saying you're undecided, or there's not convincing evidence or even that the balance of evidence indicates there's probably no God is rational (though I dispute the veracity of that last example, obviously). The inability to prove a negative, however, is a cornerstone of logic, so claiming to have done so is automatically irrational.