if he would have only said "I think it's a good idea, but honestly I don't think it goes far ENOUGH".
Think about it. If the program Obama put into place on Friday is basically the same as Marco Rubio's version of the Dream Act, then Romney would not and could not be seen as wavering if he had ripped it right out of Obama's hand in that fashion.
As far as the notion of it not going far enough, promise the world! After the election, there's no rule that says you actually have to ACT on your campaign promises.
As proof supporting the above paragraph, I give you one Barack Obama.