The first "announcement" of the Green–Tao theorem on arbitrarily long arithmetic progressions of primes was the appearance of their preprint on the arXiv. When I saw that preprint, I posted an article to the USENET newsgroup sci.math, somewhat incredulously asking whether this was the first public announcement, and Tao replied:

It is our first public announcement, yes. Given the track record for announcements for well-known conjectures in number theory, it seems a low key approach is appropriate. :-)