Stephen Hawking says although life almost certainly exists elsewhere, we should not attempt to actively initiate contact with extra–terrestrial life forms. As Jared Diamond has pointed out before, historical evidence of encounters between humans, and biological evidence of contacts of different groups of animals, shows it always goes badly for the native population, often resulting in extinction of species, or subjugation and slavery amongst humans.
Unfortunately for us, at least one group is scientists have taken it up on themselves to try to make earth orders of magnitude more visible in the galaxy. Their rational is that all advanced lifeforms (capable of inter-stellar travel) would be altruistic.
A nice discussion of this topic can be read here:
And an editorial in Nature discussed this in 2006: