In General, there has been coming and going of ET’s already. But, it’s been done privately, individually, secretly, etc. and not meant to be made public. Currently, we have so many problems on this planet, that it will be a while before a public, en masse contact will be made, because it is a fact that this public kind of event would have the effect of turning up the heat on the burners of the problems in our world and only add fuel to the problems. HOWEVER, when the new energy source comes into manifestation around 2050, they will let themselves be publicly acknowledged as being present.  At that time, there will be much which can be exchanged between humans and those that make contact public.
One of the reasons that public contact is not made, is because there would be a clamoring of the people of earth for them to share their technologies.  There would be both support and opposition to this in our social structure and if it were to happen at this time in our lives, it could have devastating effects on our economy and our social structures.  The upheaval is not what they seek to create, so they will wait until we are advanced enough to be able to integrate their presence into our world in a more peaceful and non-chaotic way.  (Added 6/11/07)

2032-2034 there will be a high amount of ET activity. (Added 8/10/05)