That is the key thing to me as well. I would be -very- surprised if earth was the only planet in the universe that has intelligent life, and if so, very ineterested in why this would be the case.
It could just be how much Sci-Fi stuff I watch, but I sometimes consider the possibility of Earth being the only planet in the universe with life on it, and it always gives me chills from the potential implications.
Scenario 1 is usually along the lines of, if theres no life anywhere else, what could possibly have happened around this plain old 3rd generation star to cause life? There is theoretically a limitless amount of stars and planets with an infinite number of configurations that could form. What is it about life that in 14 or so billion years and trillions of solar systems it only happened once, here?
I don't believe in any deities, but those odds are probably impossible to figure out.
The 2nd scenario is, that there was more life spread throughout the universe at one point, but it was wiped out. Which itself would raise a number of questions. What could wipe out every living thing from the universe? (except us). Why are we alive if all life elsewhere was destroyed? We're we around when it happened, and something allowed us to survive? Or was life wiped clean long before our planet even formed, in which case, why are we the only life to return?
Bah. Its not something I can put my finger on, but the idea of use truly being alone in this massive universe is haunting somehow.
The last VSB-TV, AM and FM broadcast transmitters are shutting down as we speak, and probably very few if any will remain operational in 20 years. This means we have been visible in the RF spectrum for about 100 years. It also means we would be completely invisible to a society only 100 lightyears away if they were lagging our technological progress by only a century, a minute amount of time compared to how long humans have been around.
While its true out radiosphere extends out around 100 light years or so, you have to also take into consideration that at that distance any of our broadcasts would likely have been drowned out by the cosmic microwave background. Remember, with sources like radio the energy density drops exponentially with distance. So take 1 KW of radio energy, and stretch it into a sphere 100 light years across, and you can imagine how thinly that radio energy is spread.
I cant recall the source, but I remember reading somewhere that the manner most of our radio is transmitted and because of the exponential drop off, you would be lucky to hear any Earth transmissions beyond a light year from Earth, with the exception of communications directed towards probes and such, since those communications are broadcast into space with a much tighter "beam" and thus the energy density would remain higher, though you would have to be in the path of the transmission to receive it.