Expats

Lately I've seen a lot of the use of the word "expat" to mean people who move from one country to another. When I first saw the word, 10 years or so ago, I thought it was just a British word for immigrant/emigrant. Nowadays I see it used a lot by Americans too, and the usage seems to have differentiated from "immigrant": the people who came from the third world to the civilized world tend to be called immigrants and the people who move from one civilized country to another tend to be called expats. Also in historical contexts it's always "immigrants" and never "expats".

Has the word "immigrant" gotten so many negative connotations already that people need to find another word for "nice immigrants"? Or is there something else at work?