Okay, I'll buy the "notice in advance," and there probably is a clause with Iberia. Makes sense. HOWEVER, I don't think you guys are thinking this through. How many deaf people do you know? Not "oh, that guy that lives down the street is deaf." How many of your friends are deaf? Personally, I've known several. If the FAs are looking at them, they could be whispering or shouting. Those deaf people are gonna know what they are saying. They've had several years (and some of them their whole lives) to adjust to a soundless, voiceless environment. To say they won't be able to follow instructions just because they can't hear and there's no one around that can sign to them is just ignorant. It's like saying blind people walk into walls because they can't see, or people in wheelchairs can't play sports because they don't have the use of their legs. 
So, while I agree that if a policy was in place that the airline should have been notified about a larger group with a disability, I do NOT agree with what a lot of you are saying about an emergency situation. If the airline booted them because of that reason, then that appalls me. If they denied boarding due to an airline regulation that wasn't followed, then I support that. I haven't heard both sides totally yet, so I'm not condemning the airline or the passengers.