This subject was touched on in an earlier thread on "why not broadcast our presence?" but I thought this article was a good roundup of the whole issue. Includes an interactive Drake Equation device!

All these factors aside, there is one additional daunting obstacle which complicates any effort to tune in to intergalactic radio. Even if the universe is thick with signal-slinging civilizations, including some old enough that their indiscriminate electromagnetism has had sufficient time to reach Earth, not even the most massive and sensitive equipment of science is currently capable of plucking the signal from the static. When any non-focused electromagnetic signal is generated– such as a television broadcast or a cell phone conversation– the energy propagates as a spherical wavefront at the speed of light. When a sphere is doubled in diameter, its surface area increases by a factor of four; but in a spherical wave the "surface area" is the energy itself. This means the signal's energy is spread over four times more area at twice the distance, resulting in a 75% loss in intensity. To put it another way, in order for a broadcasting tower to double its effective range for a given receiver, it must quadruple its transmitting power.

To demonstrate the degrading effect of distance on an everyday omnidirectional signal, one might imagine a spacecraft equipped with an Arecibo-style radio receiver directed towards the Earth. If this hypothetical spacecraft were to set out for the interstellar medium, its massive 305-meter wide dish would lose its tenuous grip on AM radio before reaching Mars.

Somewhere en route to Jupiter, the UHF television receivers would spew nothing but static. Before passing Saturn, the last of the FM radio stations would fade away, leaving all of Earth's electromagnetic chatter behind well before leaving our own solar system. If a range-finding radar beam from Earth happened to intersect the ship's path, it would be observable from a much greater distance; though its short duration and smooth, Gaussian meaninglessness would make it an inconclusive detection– much like the Wow! signal and Radio Source SHGb02+14a. A highly focused beam such as that used to communicate with space probes would also remain detectable for some distance beyond the edge of the solar system.

If, hypothetically, A) a race of extra-intelligent extraterrestrials happened to reside in the nearby Alpha Centauri star system, B) they happened to broadcast a 5 Megawatt UHF television signal, and C) we were fortunate enough to be pointing the mighty Arecibo telescope directly towards the source when it arrived four years later, we would still be unable to enjoy the zany capers of the Alpha Centauri equivalent of Mork & Mindy. In order to detect such a signal from this relatively proximate star, a dish with a diameter of 33,000 kilometers would be required. Even using Very Long Baseline Interferometry to link two Arecibo-style radio telescopes on opposite sides of the planet– thereby providing a virtual radio telescope the size of the entire Earth– our antenna area would still be 20,244 kilometers too small.