1) We, the human race, are here, alive and so are millions of other life forms on this planet - the universe is immense beyond belief and I believe that there has to be some sort life on one of the many planets out there, somewhere. Whether or not that life is "intelligent" or not, is another matter.
When will we discover extraterrestrial life - or it discover us - I wouldn't even like to guess. As for how, the distances in space are vast, so unless "they" are much more advanced then we are, I would guess it would be through radio waves of some kind...
2) We have already been beaming out radio and TV signals into space for years, so that's kind of a moot point in a way. If we were to send some kind of "homing beacon" out into space, it would have to be short and sweet and something that stands out from the normal space noise (maybe just "hello" in morse code) it needn't have a meaning, we just need to attract attention, then worry about how to talk to them if we get a reply...
3) Why do I run SETI@home? I guess it's because I've always been interested in this kind of thing, and to think that it could be *my* computer that helps find extraterrestrial life elsewhere in the universe...
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