I actually think both moijk and IOWA are right here, I think he is doing it for BOTH of the reason you guys are saying.
It is obvious Steve wants to completely destroy Adobe (and replace flash with HTML5, and I think he is right, flash should have never existed it is power hungry and inefficient and not so secure, but he just seems evil in the way he goes around trying to replace it with HTML 5, as if he had personal grudges, I thought adobe was a huge part in Apples development from the biggining, anyway...)
But I am also sick of seeing bad ports of application (not just on the phone platform) because of lazy coders, and I highly agree with what he is doing in that extent (although Im sure he does that NOW, knowing he has such a huge user base he can enforce that without the risk of every developer leaving, to protect and keep his apps on his platform, and as a company that needs to protect its self, he is right in doing that.)
The whole 4.0 announcement is because Apple is aware of android taking over, and if they do nothing, they are killing themselves. He is taking the right decisions I think, although i still think he is evil in his doing, but then again, I dont know him.