With DS, we told people pretty much immediately. We were living abroad, and planning to be there for several more months at least, but the pregnancy necessitated us coming back to the States right away (not by my choice, though).
With #2, I wanted to wait until after the first trimester and have some time to savor the news between me and DH, especially since we told everyone right away the first time. But someone at a family Thanksgiving dinner (~8 weeks) jokingly asked when we were having another baby, and silly DH and I just broke the news right there. I told people at work at about 12 weeks. I wanted to wait to make a general announcement until we heard a heartbeat and the highest risk of miscarriage had passed.