I see where you are coming from, but I really think Apple is going to have something set up so it's a very seemless easy process. Also, if you give people 2 ways to do something - one old and standard and the other new and consequently harder to initially understand and use - people will stick witht eh old one (epecially if the new technology does essentially the same thing as the old without any obvious advantage). By only giving you the new and setting it up so you don't have to choose anything and making it very seemless, it guarantees people will begin using your new technology. Although I understand that makes the purchase a little more risky, hopefully Apple can pull through on it.
Personally I'm not worried because I'm waiting until at least September to get one so I'll know the full deal by then and I can make a much more informed decision.
One of the biggest issue I can see with using email over SMS is the fact that you need to know which carrier the person you wish to send a message to is using and how this company deals with sms.This is not as simple as it sounds because you still need to figure out how which "email address" this company is providing to its customers. For instance, do you know if virgin mobile is using @virgin.com or @vmobile.com?
While this may not be a problem when dealing with close friends, it makes things a lot harder in other cases when all your provided with is a phone number.
I'm all for new technology, but as you say, I think the transition needs to be as seamless as possible. I think offering some sort of compatibility mode with the previous industry standard, for those situation where you really need to use SMS would have been a great compromise.