A steel block ("iron" block, whatever) is less prone to warping if it overheats. But as far as I know, thats really the only advantage anymore. IMHO, the weight advantage of using aluminum is more than worth the tradeoff for the increased risk of warping. After all, you can counteract heating with a larger capacity, reliable cooling system. I think racing engines are pretty muuch aliminum anymore anyway.
Who would want to cart around an extra 500 or so lbs in their Celica just to have an iron block anyway?