I always thought that this phrase (as a meme) originated with Admiral Ackbar. In a recent thread though, someone said it came from somewhere else. I can’t find the thread, and I don’t remember the claimed origin.
A little help?
I’m amazed that anyone thought that phrase originated with Star Wars. Surely it’s a staple of every World War 2 flick where the fighter pilots get suckered into an ambush, which the gloriously win, though not until after Butch and Brooklyn get shot down.
I’m talking about the Internet meme.
I always associated the phrase with the joke from the 6 Day War. Although I’m sure it dates back to the Romans, at least.
It was actually a misprint in the script, and George Lucas decided to run with it.
Here’s what it was originally supposed to be.
According to Wikipedia, it (the internet meme) started over in the somethingawful.com boards.
Now that’s funny, even though I’ve heard only the punch line. 
As I understand it (and the purported Something Awful origin would seem to back), the “It’s a trap!” meme started with nerds on the internet jokingly referencing the Star Wars usage to warn of transgendered porn.
IE, photos or anime where it was not readily apparent the individual is a guy; hence, it’s a trap.
A more homophobic meme, from a more juvenile age.