From a special section on Astrobiology in the Feb. 22, 2001 issue of
Nature:

Where are the dolphins?
Nature, Vol. 409, 22 February 2001, pp. 1119-­1122
http://www.nidsci.org/articles/pdf/nature_v409.pdf

"For similar reasons, it is probably pointless to search the heavens for radio signals from other worlds, as the Search for Extraterrestrial Intelligence (SETI) project aims to do [18], [19] (see refs 18, 19, and the review in this issue by Wilson, pages 1110-1114). It would be equally sensible to look for smoke signals. Radio did not
exist on this planet a hundred years ago, and might become obsolete. If aliens communicate at all, they might use media as yet undiscovered by human technology. Even if radio were their medium of choice, they might have encoded their transmissions for optimal efficiency. Moore [20] has shown that an optimally efficient coded message will be
indistinguishable from black-body radiation. Imagine a Second World War radio operator picking up one of today's encrypted satellite TV channels: it would sound like static. Is this the true meaning of the cosmic background radiation?"
p. 4

"However, despite their seemingly limited diversity, Earth's current life-forms may be more typical in other, more important ways, such as their relationship with their context. 'Life' is a name we give to certain emergent processes of complex systems [22] (refs
5, 22). Until quite recently we used the word as a catch-all to cover anything on this planet that seemed to have some kind of individual autonomy. It then became evident that everything of that kind was
using the same trick - DNA (or RNA) and associated biochemistry. We have therefore assumed that DNA is the sole route to autonomy and self-complication. However, the prevalence of the DNA mechanism on
this planet may be just a historical accident. When any one such trick evolves, it quickly dominates - the trick, by its nature, is self-copying, and tends to swamp the competition.
"None of this implies that alternatives, especially radical ones, cannot exist. For xenobiological purposes the answer to 'what is life?' cannot be a catalogue of DNA bases. It must involved the recognition that the abstract processes of life possess certain universal features, and that those features might have a large number of possible different physicochemical realizations."
p. 5

Interestingly, there is evidence that this last may be the case:

Scientists Are Starting to Add Letters to Life's Alphabet
The New York Times Health Section, July 24, 2001
"SAN DIEGO, Calif. -- Scientists are taking the first steps toward creating alternative life forms -- organisms that use a genetic code different from the one used by all other creatures on earth."
http://www.nytimes.com/2001/07/24/health/genetics/24DNA.html

Engineering ET: The Path to Alternate Life Forms
"In two separate research efforts, scientists have altered the very nature of nature by creating cells that break a cardinal rule of biology, incorporating an entirely new basic building block into their cellular structures. It is a first step on a path of neogenesis -- the creation of alternate life forms."
http://www.space.com/scienceastronomy/generalscience/neogenesis_scitues_010501-1.html

Bob Clark