SETI has now established a protocol to disseminate news of such a discovery, which basically breaks down into 1) confirm, and reconfirm, and reconfirm again before you say anything; 2) go through channels; and 3) no one should talk back to them until a public international discussion is held. This may all be a moot point since people have been sending signals in various directions for some time, and a criticism of Frank Drake for doing just this is mentioned in the paper I linked to (references removed for readability):
Such a signal was in fact sent out by Frank Drake in 1974 and Ryle wrote to Drake complaining that it was "very hazardous to reveal our existence and location to the Galaxy; for all we know, any creatures out there might be malevolent - or hungry". Later, it seems that Ryle led an approach by several people to Sir Bernard Lovell of Jodrell Bank fame who then sent a private letter to the International Astronomical Union raising the possibility of malevolent aliens, saying that "I have been asked to seek a discussion in the Executive Committee ... astronomers are involved in the problem of communication with extraterrestrial communities. Transmissions for this purpose are being made .... [ as to whether] the IAU should draw the attention of world governments to a problem which could conceivably be of critical importance" and "whether the astronomical community should take steps to initiate a wider discussion on an international basis of the consequences of success ... I repeat I raise this issue on behalf of a number of distinguished individuals". After consulting Drake, the IAU concluded that no action was needed.