Now I really think there will be a lot of post on this one simply because everyone has run across the ostrich mentality when discussing alternate science. People who do not believe have three reactions to alternate science:
1. Polite- they appear to be listening to in fascination as you recount all the interesting evidence out there for alternate beliefs but...... as you speak to them you can actually see them shut you out of their thought process. They appear focused but begin to give you pat answers they heard on the tv or what they think you want to here. That is the give away and it is also your opportunity to end the conversation early so you don't waste your time(and apparently theirs too)
2. Hostility- your presentation of alternate theories may provoke some of the more programmed inividuals in society. To contradict things a person holds as gospel can insight some strong emotions in those not equipped to think outside the box. The give away is easy bulging eyes heavy breathing and pawing at the ground are the obvious signs, you might run into sum low level debunking that lacks a cohesive argument and just basic flailing about in panic. Avoidance of these automatons is essential to your sanity they will drive you crazy with their inability to learn anything more than what they were taught in school.
3. Fraudulent belief declaration- These are the most dangerous of foes that you can face in the alternate science venue. They profess the same beliefs as you but are really wolves in sheeps clothing. They are usually the ultra conservative believers (i believe but only if its right in front of me) type. They talk a good game but are just that, talk. Fortunately for the true the believers these people are easy to spot.. A fraudulent believer will give himself away by never stating a belief or opinion while criticizing everyone elses statements. They are the type who likes to prove how "smart" they are by regurgitatating commonly known facts as gospel like they were the first to state them. But they do serve a purpose
Now what do you suppose these types of people would when confronted with undeniable proof, you tell me.

Mass hysteria.

It will probably be 2 and 3. (mixed)

But for many it will be 1.

This is the most intelegent thing I've seen here in awhile, and well put. I agree, and I can name a few people I know in each catagory. I'm surprised Kismet hasn't locked this thread down yet like my hunting big heads. Your entirely correct and those in the closed mind catagorys will get a shock soon I think. But expect advisaries right here to attack your true observations. Why? Because they have closed minds.

UFO's have already entered the mainstream. You mean their occupants and their technology intergrating with ours and our cultures.

We already have diverse cultures and species on this planet and our technology is growing by leaps and bounds.

It will prove interesting not on their reaction to us; but the human race reaction to them. Look how we treat each other now; G-d only knows how we are going to treat them.

The human race would invite them down just to blow them up. I would just keep watching from above, or send in replicates to mould in and not be recognized to not create paranoia.

I think we have been "conditioned" for many years now.
Even though I've believed we are being visited since I was
very young, it's still hard to get your brain around the reality
of standing there looking at an alien being. It would truly have
a profound effect on everyone.

So, do you mean when the truth is on the news? or when
aliens actually show themselves?

I take it by "mainstream" that you mean on the news. This
would definitly change a lot of things. Besides the 3 basic
behaviors already listed, I think there would be a lot more.
Does anyone think that it would change our way of life?
What about all the social, political, economic, religious, etc;
implications? Any ideas???

The worst of it would be the religious, for the sheer importance of a revelation that aliens exist would mean to them.I don't know the exact numbers but i seem to recall that the current statistical number by guesstimate worldwide of people who believe in some sort of god was something like 4-5 billion. Now of those people its hard to quantify how many are true believers or just hedging their bets, but, its probably a big number. For them the discovery of aliens could turn them inside out. We are talking about a group of people who believe strongly in the life of intangable beings as a reality so when it comes to rationality they come up a bit short. You might expect to see these type of people react like its the end.
All in all the religious factor will be the kicker, all the other things are managable because of the global community that exists today.

I remember once, when I was visiting Chile, my grandmother asked me if I had ever met an alien. I thought, at first, that she was asking if I had seen a UFO, but it turns out that she, and a good amount of the people in the town she lived in, where under the impression that the events in the old TV show "Alien Nation", where derived from an actual event and that there was a sizable alien population living in Manhattan.

These were not ignorant hillbillies; they were simple townsfolk who, I would have little doubt, would probably react to an alien presence by inviting it over for tea.

Interesting. Those are normal people, we are not. We are online, mutated, self-rightious, scared, poisioned, judgemental and dangerous. We are changing for the worst, inside ourselves. I'm a mutant too. And I know it. I think a great awakening could be the only thing to change our ways. Back to the good old days.