I just checked and in my case I definitely don't have to press and hold, just firmly press.
I can actually feel the difference when I am pressing it firmly enough as there are two "clicks" of sorts. I'm assuming the first one is the rubber making contact with the physical button, and the second is the button actually depressing. A light press only results in one "click" and the button barely depresses. A firm press though and you get both clicks with the button going down pretty much flush with the side of the phone.
With the 9000 there is only one "click" and even the lightest of presses does it. Also the button itself is a lot shallower per se. Both the result of it not having the added layer of rubber, I'm assuming.
Goodness, all of that for a button - much easier to feel than explain