There are more ways than one of killing a cat, and the same is true of
a radio signal. As far as I can make out, the sort of signal we are
expecting to hear from our fellows in the universe are pretty old
fashioned, even by our current standards.

Many communication systems these days use spread-spectrum techniques
in which its really very hard to detect the presence of any sort of
signal unless you know exactly what you're looking for. The
information being transmitted is spread over a wide range of
frequencies so it looks like a very general slight increase in the
noise level. Unless you know the spreading function, Its pretty near
impossible to detect that its being transmitted at all. One of the
advantages of is that if you code it right, its possible to recover
the original information even if parts of the spectrum are obscured by
noise. There are several other significant engineering advantages
which is why its becoming more popular.

Also, I think that a efficient coding of information would certainly
sound like white noise anyway unless you know how its being done. If
you listen to an old fashioned 300 baud modem, you hear tones being
modulated. Listen to a modern V92 modem, for instant and it sounds
like white noise.

As a result, although I run the SETI at home screen saver, I'm far
from optimistic that anything will be found, even if our friends are
actually sending. I'd love to be reassured that the signals ARE being
examined for more exotic modulation tricks, by the way.