Regarding the "end of the world" in 2012, I don't think it is going to happen. I think Mayan prophecies have been largely misrepresented, but this is a complex topic.
Regarding an "encounter with aliens" in 2012, after my study of ufology, I think there is evidence (not conclusive, but more or less good and interesting) to think some beings outside the Earth have been in touch with us for a long time. This could include beings from other planets, or from other dimensions (in both cases, they could be considered "aliens").
I don't discard that a "open, public contact" will happen some day, but I see no reason to think it will happen in 2012.
