It probably depends on the datacenter environment and the quality of machine you are buying. I'm not talking about logging on to godaddy and asking for 75 random dedicated servers. I'm talking about going to Dell and buying 75 machines and putting them in a rack. You will know exactly what you are getting. We use a datacenter that is also a Dell service center so they have parts on hand for any failures and they can service any Dell machine that comes with the base warranty.
With 75 good quality machines I bet you wouldn't see more than three hardware failure a year if that. All 3 of those would probably be drive failures. I imagine the redundancy of the software would handle that without a problem.
I agree that the continence of EC2 is very nice. We use EC2 a lot but just in a lot more "elastic" way. We have found it is cheaper to colo boxes if the demand is constant.