Your immediate thought when reading the title is probably one of the following: Spaceships, UFOs, Abduction and Conspiracy Nuts. However bear with me for a moment even though this does get a bit heavy, I can assure that I have approached this thread with as much critical analysis and scientific method as I could muster, while still being a Philosophical nut-job!
Something occurred to me while standing on my balcony having a cigarette and thinking about an article I read on the Beeb.
Hypothetically speaking our technological progress is set to continue at it's current pace. It is in reality exponentially increasing pace, but for arguments sake let's say it continues at a steady pace (you never know what hiccups are around the corner!). We are already able to view, and take pictures of some Exoplanets that exist within our Galaxy. It is not that much of a leap of the imagination to assume that as our ability to view the surrounding Universe expands due to our technological development, as will the resolution and detail of these pictures and also the ease at which we are able to take them.
Both Drake's equation and the Fermi Paradox propose differing ways of looking at whether extra-terrestrial civilisations exist within the Milky Way. Unfortunately, I am an optimist in this respect. Considering the relative size of our galaxy, the distances separating stars, and the recent discovery that photons cannot travel faster than the speed of light (thus holding up Einstein's theory of relativity), I'm not surprised that up until this point in our technological evolution as a species, we have not been able to find evidence to support sentient (or even primitive) life from within the confines of our Solar System. By this I mean, Radio Transmissions from other planet, Evidence for life on Mars or even UFOs crash landing. Basically the stuff we can look at with post-war technology and without the help of the advanced technologies that the computer revolution of the past two decades has started to produce.
So for the sake of this ponderance, I ask you to assume (if you don't already) for just a moment, that we are not the sole sentient life in our Galaxy. We should consider that up until this decade, human beings have not been in the technological position to photograph Exoplanets, and it is not much of a stretch of the imagination to assume that within another decade we will start to take really in depth and detailed pictures of other Planets. Photographs that allow us to see cloud formations, seas of methane, volcanic activity, plant life, and even intelligent life. This is itself is truly remarkable, and will be the turning point for finding out whether we are truly alone in the Universe or not.
So now I'm going to turn this on it's head. If we're not the only intelligent species in our Galaxy, and considering the age of our Galaxy, who's to say that those other intelligent extra-terrestrials aren't ahead of us technologically? They might even be far ahead of us technologically. I personally would like to believe that relatively quick travel between stars is possible, but that's a personal belief, and rationality and logic, and the current scientific understanding of the Universe tells me that it simply isn't possible within human life expentencies. We cannot travel faster than the speed of light. It's physics. So I can understand why Aliens won't have been able to visit us.
But, there's no reason that Aliens of the future, haven't got us under constant watch, or perhaps have turned us into some sort of birds-eye-view Big Brother variant. Light takes time to get to from star to star, hence the term "light year". Some of the stars that you can view up in the sky today have since gone Supernova and burned out into a white dwarfs, neutron stars or black holes. But in a million years time, light from the Earth might reach a Solar System home to a sentient species, who might still be evolving in the mud now when you read this post, but will then be a Sentient culture in a million years time; with technology well beyond this primitive Quad Core PC I'm typing on right now and with the capability to capture photos in real-time of our little blue dot. They will have the ability to do what isn't that beyond us right now, look at us, study us, and know that they are not alone in the Universe.
So, there's me, standing outside with no cloud cover, now uniquely aware of the fact that aliens could possibly see me unshaven, and with messy hair, thinking, "Oh shit!" :P.
In all seriousness though, bring on the next 20 years. I'll bet anyone a fiver we'll find ET before I turn 45! This is an exciting time to be alive. In fact, this post might have just pushed me towards doing a degree in Astronomy. It really is fascinating stuff.