It all depends on the situation and context. When I left my previous employer, I told them exactly where I was going. The mentality at the previous employer ha

It all depends on the situation and context. When I left my previous employer, I told them exactly where I was going. The mentality at the previous employer had been changing (post-takeover), and the new company was founded by, and mostly consisted of people who came from the old employer and disliked the new direction. Everybody at the new employer knew what the old employer was like. If the old boss would have made such a call as you describe, the new boss would have seen right through that. In the case of the resignation in the link, I suspect the programmer left on amicable terms, which is also a situation where you could tell the old employer where you're going. As a rule, you never say "never ever" :-)