We discussed this quite a bit on the ancient alien thread. There is also a book with over 100 reasons why we haven't encountered aliens yet.
My most likely scenerio: Aliens are using Starseeds, self-replicating robots that are cheap and efficient...virus-like, and may well have settled decent portions of the Galaxy, but even though advanced, tiny robot probes are almost impossible to detect(we can barely find asteroids that pose a danger to us).
I also lean towards the idea that any alien species advanced enough for star travel will likely not be malevolent, though they may well have practices that would endanger us, but they may indifferent to such problems.
I have a feeling that our methods of communication may not attract much attention from space travelers, however, if thorough enough, our artificial signals may be picked up, but it just won't be the prime method they are looking for.