For those interested, the forthcoming films "Battle of LosAngeles" (taking the name from a real and unexplained WW2 incident) and "Monsters" also deal with the subject. The latter can be seen on I-tunes and OnDemand already and takes places some years on from the aliens being here. The former is ground level "Blackhawk Down" style combat between the USMC and the aliens in Los Angeles.
When asked about what to do if aliens invaded, the official response circa the time that Independence Day came out was to "Dial 911". Not the most reassuring answer, however...
Plans for such things do exist. They aren't necessarily good plans mind you, and some are out dated, but they exist. (The same as if Canada invades, we'll dust off the old war plans from the 1930s I suppose.)
Information to the effect of what to do actually made into some fire fighting manuals approved of by FEMA for a time.
First responder wise, there's also the matter of dealing with outbreaks from alien bacteria surviving a meteor ride to earth.
The grays and reptoids meanwhile continue doubtless continue their war against the Nordics and time traveling space Nazis. The reptoids might not even be aliens per se, as they could well have evolved right here on earth long before mammals came along. To them, we're sort of the "Planet of the Apes" future I'd imagine.
Dr. Drake of "Drake Equation" fame was still alive last I knew. That's who's numbers Sagan used to crunch.
There are a number of ways, even with the physics we know today, to either travel FTL or eliminate the need for such.
Things become even more complicated if the multiverse theory is correct, and travel between alternate universes becomes possible. Then it becomes open to debate whether or not someone from a variant earth is an alien or not.
And that Roswell NM stuff...My old Flight Surgeon from my Army Aviation days...His Dad was the Intel Officer at Roswell when the Roswell incident took place. i'm a believer after speaking to him.
Jesse Marcel Jr? I can't remember if he's still in the reserves or not. He actually made it all the way up to Brig. Gen. if I recall.
When UFO Hunters put him and a guy that had claimed to pick up debris at the time (and who records showed was certainly there) in a room and had them look at metals and such, they both picked a type of mylar as being very close to what they remembered handling.