i think aliens will make their existance known to us when we can show maturity and responsibility as a race of people.
kinda like letting a kid drive your car. you ain't gonna give a 16 y.o. the keys to your benz.
also to quote Eddie Griffin:
"aliens ain't comin here. bad news travels fast. they prolly saying 'they fightin over black and white, shiiiiit we're purple, WTF you think they'll do to us?'"