There are so many telescopes and specialist astronomical CCD cameras pointing to the night skies that it would be impossible to keep an asteroid discovery secret. The vast majority of telescopes capable of finding and tracking such objects are actually NOT in government hands or control.
The +Slooh Space Camera
telescopes are an ideal example - anyone can use them, point them anywhere they wish, and receive images in real-time so they can't be "intercepted" by any other party.
Whether NASA and/or world governments would choose to share any information regarding an impactor, one thing is for sure - it wouldn't stay secret for long. In fact, it's likely that amateurs would make the initial discovery - and we are extremely well practiced in disseminating information across the Internet in a matter of minutes, and in a way that could not be stopped.
It's not NASA or governments we should be afraid of - it's those large lumps of space rock!