I would first of all like to say that I agree with jamin. You jump too quickly to conclusions. While this object that David saw may have been mysterious, I find it much, much more likely that this was some sort of military aircraft. Cover up it may be, but an alien one, almost certainly not.
Don't get me wrong, I definately believe in aliens, but the chances of us ever coming in contact with them, given how immense this galaxy alone actually is, is doubtful.
The entire thing about alien aircraft at area 51 was a government hoax. They were researching stealth fliers to spy on Russia, and they suspended the belief of aliens to keep what they were really doing a secret.
The problem with the belief that the government is covering up alien landings is the amount of effort that would actually have to go in to a coverup. You are assuming quite a few things...

1. The aliens would land in the U.S.A.

2. No non-affiliated observatories would pick up some kind of light or something when the aliens came through the atmosphere.

3. The government would be able to prevent anyone from getting any conclusive footage ever.

4. The aliens are always cloaked in mystery, staying just on the edge of being seen. Never trying to make actual contact, but rather "abducting" people.

That's pretty much all I wanted to say.