Well, not so fast.
Every planet have a vast night sky in which to search for signals. The further away they are from our solar system, the harder it will be for them to pick up a signal; not only will most have to have their finely tuned directional electronic ears pointed right at us, but their equipment will need to be able to receive the frequencies that we are radiating out into space with as well as be free from possible local interference. It is possible the aliens could dismiss, at first, our signals as noise.
The aliens will only have a small window to detect the signal as their planet rotates on its axis (unless they have space based radio telescopes). Interstellar dust could attenuate some, or all, of the signals. And finally, if they pointed their radio telescope to the Earth a couple of hundred years ago they would've not heard anything and may take some time to come back and listen again; or they may discover artificial (unnatural) signals from one or more other star systems and that may be enough to preoccupy them, delaying them from continuing their search, or from their going over previous searched sections of the sky. So even if the cat is out of the bag, we may have time to put it back in.
So, we need an alien race close enough to pick up the signals we've been leaking, with the technology to pick up the signals and to recognize them as non-local artificial signals, and the will to take the time to dedicate equipment for meticulous searches of the vast sky looking for alien signals.
Even if they did find our signals, they would have to then figure out what they are. Radio and television signals are not simple signals - they are signals which are modulated onto carrier signals. They will need a receiver that is capable of distinguishing signals from each other, able to remove the carrier to retrieve the information being sent, and then process it so that it can be used by a speaker, or TV screen. However, the original signals are meant for human ears and eyes, not an aliens', which could further complicate their ability to receive, and perceive, the signals.
If they are able to figure out the signals, and are able to get the information out in audible and/or visual form by adapting it to their own hearing and/or visual range, they would still need to interpret what they are seeing. Being that our culture and theirs will be very alien to each other, it may be profoundly difficult for them to make sense of what they hear or see. They may not perceive music as we do. They may not even be able to hear, communicating instead through scents (like ants), or touch, or light. How would such aliens make sense out of a audio broadcast? Will they have a sense of humor or be able to understand ours? Will they mistaken a TV show as a historical document (like the Thermians in Galaxy Quest)? Will they be offended by what they see either because of cultural differences or because of misinterpretation. It's hard to interpret a truly foreign language, especially the context, thousands of light-years away.
(I have to pause for another humorous aside, this one from Futurama - will they become addicted like Lrrr, the ruler of Omicron Persei 8, to a 1,000 year old TV series and demand the last episode be shown or the Earth is destroyed?)
But it may be enough that they detect the signal, and realize it is artificial - they would be able to figure out where they came from. If the speed of light is the ultimate speed limit, and there are no "tricks" to get around it (warp engines, worm holes), there's not much to worry about - physically (theologically and psychologically we may have something to worry about, as mentioned in earlier posts on alien contact).
Anyway, while the amount of signals we're leaking out into space has increased (cellphones, space probes, GPS, for example), weakly leaking out man-made radiation isn't the same as beaming out a strong focused signal on purpose for aliens to... oh, wait, we've done that. Ooops. it looks like we not only let that cat out, but we sent it running, meowing all the way.
More on that in the next post.
"Character bios: Lrrr." Can't get enough Futurama. 6 August 2008. <www.gotfuturama.com/Information/CharacterBios/lrrr.dhtml>.
"Galaxy Quest (1999)." Internet Movie Database. 6 August 2008. <www.imdb.com/title/tt0177789/>