Okay I have a non techy answer.
We are searching the data spectrum for signals in a specific mhz
range. This mhz is the one that, I believe, Hydrogen atoms vibrate.
Meaning that any communication recieved would be from life forms that
have Hydrogen based life, like ours. Yes that means that "some"
technologies won't be found using this technology, BUT it does mean
that civilizations that are founded in Hydrogen based life forms,
which is where the vast majority of life as we understand it would be,
WILL be found, eventually.

On 4 Feb 2004 05:44:19 -0800, kpwatson@luna.co.uk (Keith Watson)
wrote:

I've been enthusiastically running the SETI@Home client for several
years now.  The other day my wife stopped me dead in my tracks with
the throw away line "Doesn't that assume that all technologically
based cultures always use a specific part of the electro-magnetic
spectrum to communicate, etc.?" (or words to that effect, I'm
paraphrasing a bit :o) ).

It did get me thinking though. Although our current technologies use
electro-magnetic waves for communicating (i.e. radio, television,
radar, etc.) are we justified in assuming that we will always use
this? and by inference assume that extra-terrestrial intelligences
(ETIs) will also?

Trouble is we can't predict the future.  Think how different the
technologies around now are from those around at the turn of the
19th/20th centuries. There are many we have today that weren't even
thought of then (indeed I suspect a number of very learned and
educated individuals at that time would have taken great pains to
prove that they were impossible if you'd have even suggested them). 
It's entirely possible that some bright spark (no pun intended) will
come up with a new comunications technology that doesn't use the
electro-magnetic spectrum at all. And if so the chances are that other
ETIs will also.

I have this uncompfortable picture of us busily listening for
electro-magnetic signals when evidence of ETI is all around us but we
just haven't the technology to recognise it.  But then, if this is
true, then all the other ETI's probably go through the same phase! :o)

I suspect that, until we know better, we just have to keep banging
those rocks together.

Regards,

Keith