It's happened a couple of times on mine, keys getting stuck and not bouncing back. I was speaking to a apple tech guy and he said to return it and they would replace the keyboard etc but it was at a time when I had to use the machine daily so couldn't do it.
Touch wood I think it was just debris under the key which after flipping the thing up and hitting the key numerous times it seemed to do the trick. With the keyboard the way it is, even the smallest crumb can be an issue as there are practically no tolerances.