happened to me...
I had this problem with the "c" key on mine. The apple store replaced the key and it worked fine for 4 months... but it never appeared to sit quite in the same plane as the others. After 4 months it broke again, and this time apple replaced my entire keyboard (i have apple care). Unfortunately, the keys are being held up by two very, very thin pieces of plastic and I don't think it takes much sideways force to snap them, particularly if you get some 'foreign matter' under the keys.