There's another possible problem with contact: Time vs. duration of existance.
I was out to a 4th of July fireworks display over that weekend with an astronomer friend of mine, and we got onto this very subject. And their explaination of why we haven't been contacted was very persuasive... She waited for the fireworks display to set off one of those "bomb" type mortars... you know the ones, they mostly consist of a bright flash and a bang that makes your teeth rattle.
"There. Right there! Imagine that as being the average lifespan of an intelligent civilization! Bam! By the time standards of the universe, that's all we, or any one else adds up to: a flash and a bang!"
We waited several moments, then a second one was set off...
"Although they happen, they almost never go off at the same time... they are seperated by millions of years. Just like stars themselves, even if we were to detect one, that wouldn't mean they'd still be there by the time we established communication over thousands of years."
Another pause. They set off crysanthumums, colorful burning rings, shooting stars... but it was a good five minutes before another bomb mortar went off...
Number Two: Conform, Number Six! Conform!
Number Six: I will not be stamped, filed, indexed, briefed, debriefed, or numbered! I am a person.