It's not necessarily so, however. Now that I actually think about it, I'm not so sure that they would be. We say that the natural world (Animals, plants, etc....) were tainted because they were under man's dominion - basically, God put us in charge and we screwed it up. But if we weren't in charge of these sentient aliens, then they wouldn't be affected by it.
A good argument towards that end would be the angels. There are certainly fallen angels, but there are good ones who haven't sinned as well, and weren't affected by Adam's sin - because we weren't at the time higher than the angels in the "Hierarchy of Souls". (As far as I can tell, that term isn't used by Christians, but I think it sounds cool

) So if these aliens are are equals, or superiors in this metaphysical hierarchy, then they wouldn't be affected.
It's interesting to think about, but somehow I doubt that we'll ever find out for sure.