But, Calvin, P(intelligent life contacting us | intelligent life exists) >= P(intelligent life contacting us | intelligent life does not exist) = 0, so the fact that no other intelligent life has contacted us can only be evidence against its existence.
(The problem with formally bringing out Bayes' law is that, by the time you've gone through and stated everything "properly", your toboggan will have already crashed into the brier patch.)
I think the joke hinges on equivocation of the word "intelligent". Taboo "intelligent", use "sapient" and "clever" for the two meanings, and you get: "Sometimes I think the surest sign that clever life exists elsewhere in the universe is that no sapient life has tried to contact us." Or, put more accurately, "the fact that no sapient life has contacted us is evidence that, if sapient life exists elsewhere in the universe, it's probably also clever".
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.