Having has the same problem myself (which by the way also exists using Windows XP in Bootcamp), the only conclusion can be that it's a design fault with the keyboard - I've applied countless firmware updates (there seems to be a new one every month these days) to no avail. As mentioned by others, this is not the same as the 'missed first press' problem which was addressed in firmware. It's poor hardware design (as mentioned already by others, try pressing any key on its corner - then try the same thing on any decent PC keyboard and notice the difference) It sounds like a minor problem but it's intensely annoying - particularly when entering passwords.
I find it bizarre that Apple prides itself on paying such amazing attention to detail in 95% of each product, but then overlooks the other 5% such as the rubbish keyboard here. This also happens with their other products - eg on my old iBook, the keyboard would flex upwards and touch the screen when closed which would leave an imprint on the screen. That felt like a cheap flimsy keyboard too...
I love apple, but if they keep dropping balls like this, no amount of pulsating sleep lights, backlit keyboards, magnetic power connectors or ambient light sensors will make me buy them again...