I'm surprised about that, Todd said somewhere he didn't want any dragonshouts and spells making each other redundant, wonder if the dragonshout is a lot more powerful than fire spells. Will be pretty cool to breathe fire.
If anything, I'd think the dragon shout should be weaker than spells with similar effects, since it requires neither skill to cast nor magicka, of course dragon shouts have a cool down timer, but that doesn't stop you from doing other things until you can shout again.
Though I'm pretty sure it had already been said before that there was a fire breathing dragon shout, at least, I read something to that effect here, and if it was just speculation, it was certainly surprisingly accurate, considering that fire breathing is not an ability I'd usually expect the player to have in a game where playing as a dragon is not an option.