I think they may have just switched the names around. normal hell inferno. But I could be wrong. I would love to see a 4th difficulty more than anything in Diablo III it would be a great way to make up for the lvl cap and give those that enjoyed grinding from 90-99 in Diablo II something to do. I would picture it like this.
hell - where you would hit lvl cap and gear your character up and perfect your builds, then
inferno - where you would need a large amount of gold for your costly repair builds as you die multiple times trying to figure out how to pown monsters. I don't think they should make any huge advantages in inferno over hell to be honest. rather just make it something challenging to do. Kinda like those insanse, legendary modes in other games. Just crazy hard.