In my case, I've noticed that I tend to press the Enter key towards the top, with the force going "upwards". This made it fail many times, requiring me to hit it again, harder and in the middle. Very annoying.
So... I went to the store this morning to ask if this was covered by the warranty; I had nothing to lose. And yes it was; they replaced the entire keyboard with a shiny new one without questions. Great!
Of course, I'm talking about this specific Apple Center. I do not know about the customer service in the others, but there surely are better and worse ones. (Would like to be proved wrong ;-)