Apple and AT&T are under no obligation to offer this to 1st generation iPhone users. "Should have" is completely subjective, IMO.
People keep comparing this to other phones which is absurd if you consider what people have gotten over the last 2 years in terms of updates. I am surprised there have been as many as there have been. Do other phones get multiple software updates a year? Not so much.
As far as technology goes this is the same old complaint. If you don't have the newest hardware you don't get every feature that is available. Sometimes you will but most of the time you won't. Deal with it. Computers, phones, cars, the list could go on.
I don't think original iPhone users are entitled to MMS, but it would be the decent thing for Apple to do.
The "revolutionary" iPhone was lacking a feature that had been in phones for 10 years or so. Many users bought the phone knowing it didn't have this feature because it did everything else so well... then there were those of us that didn't know it didn't have MMS until after we bought it (after all, why would I check it has a feature that EVERY phone released in the past years had...)
Now, Apple have finally created a half-decent MMS implementation, but are not offering it to the early adopters of their new device. Apple should offer it to all and apologise for not including it in the first place.
But please, keep on defending Apple for their stupid decisions like every other fanboy does.