We don't even know for sure that Area 51 won't be in the original game, so isn't it a little early to be talking about having it in an addon?
If Area 51 does appear, though, I hope the big secret in it isn't aliens, aside from the fact that aliens were already done in Mothership Zeta, it would seem far too obvious, seeing as it's already been done so many times before, I'd rather something we don't expect.
Although as the poster above me said, the alien blaster and such did appear in some form or another in older games, aliens never played a major part until Mothership Zeta, and if the complaints directed at Zeta are any indication, Obsidian should probably go back to keeping things that way.
Sorry im not from USA
i know vegas and area 51 are in Nevada
i was not to sure how close to each other they are
that was why i said add-on
but it could be in the game
the mystery parcel could be from area 51
you never know