WARNING: This post was manufactured in a facility that processes links to TVTropes and Wikipedia.
It seems quite likely that humans will eventually contact sentient extraterrestrials, not just in messages, but in the flesh (or closest-equivalent-to-flesh).
I predict that in less than a year from that day, no matter what the aliens' chemistry, physiology, cultures, philosophies, intelligences or emotions happen to be, a large contingent of humans will dig into our various subconsciouses and come up with an excellent justification for classifying them as having less-than-human status, and thusly be willing to treat the ETs as a means to an end.
The key thing is that the relevant language will not at all sound like present or past racism. A whole new set of highly reasonable-sounding ideas and phrases will be marshaled to the cause of rationalization. And the rationalization will be cleverly specific to the nature of the aliens in question, making it seem like if it were any other galactic civilization in question, then of course they'd have rights, but these ones clearly fall short of our standards of intelligence, or emotional capacity, or hygiene. These ones are probably zombies, even. (All it takes is one good authority figure asserting such a thing to make it suddenly seem quite plausible.)
So I guess what I'm saying is: I can't wait for the day when humanity has encountered two extraterrestrial species.
(Post inspired by District 9, a film I actually don't have must use for, and the late William Tenn, who I have a lot of use for. Thank you for your needed cynicism, Phil.)
Addendum: Naturally, it's entirely possible that the aliens will consider us in the same way. Just goes to show… something.