I always thought it was from that scene in The Karate Kid where Ralph Macchio is cleaning up Mr. Miyagi’s house, and has some trouble, uh, buffing his wood. Mr. Miyagi shows him how to do it and says: “See, Daniel-san? Klaatu, barada, nikto.” The rhythm of the phrase combined with the arm motions he makes in applying the wax turns out to be one of the super duper secret moves that Ralph uses to whup some ass later on in the movie.
Clearly it means: Hey kids, don’t smoke.
“Klaatu says, Retreat.”
It came from Forest Gump, when he sat on the park bench, saying “Life is like a box of chocolates, you never know Klaatu Barada Nikto.”
“Beware of my large and deadly breasts”
You are all such cultural barbarians. The lot of you.
"Klaatu Barada Nikto" is a Finnish sentence, mostly known because it was used in the dubbed-over Finnish version of Teminator II. Yes, it does mean “I’ll be back”.
Thank’s aseymayo. Geez, I post here just to get a simple straightforward answer to a simple straightforward question and these, these dirty hooligans toy with me as if fighting ignorance is some sort of game. Hmmfph!
Anyway, thanks again for setting things straight. 
Boraxo Nippon ay? ::shrug:: makes sense.
“Hello. My name is Klaatu Barada Nikto. You killed my father. Prepare to die.”
And from “The Lion King”
Klaatu Barada Nikto… what a wonderful phrase…
Hey, how do y’all know you’re even spelling it right? It could be “Clahtoo bahradah nicktoe.”
And who could forget those recent Budweiser commericials with several friends greeting each other by wagging their tongues and going “Klaatu Barada Niktoooo.”
Wolverine:
Prepare to die. No, really.
I amend my earlier answer. It’s really from the Taco Bell commercial where the chihuahua says: “Yo quiero Klaatu Barada Nikto.”
Getting back to the OP (if it ain’t too late):
Maybe, if translated, it actually means, “Klaatu has stubbed his toe,” but that’s a code. “Stubbed his toe” would mean “is dead.”
I’m not quite serious.
Not to get all serious or anything, but …
It’s interesting to think what the phrase might have meant given Gnut’s & Klaatu’s relationship in the short story the movie was (loosely) based on, “Farewell to the Master” by Harry Bates. (I don’t want to spoil it, go here for the full text (yes mods, it is reprinted with permission) there’s even a comic book adaptation on the same site) The story doesn’t include the phrase, though, since Gnut takes off once he has the means to resurrect Klaatu.
I think it was Albert Gore who came up with the phrase, “Love means never having to say ‘Klaatu barada nikto’”
I first read ‘Farewell to the Master’ when I was 12. That last line gave me the shivers.
Also, I can’t believe you all forgot that famous scene in the Wizard of Oz when Dorothy clicks her heels together three times and says ‘Klaatu barada nikto’!
And who could forget that climactic ending scene from “Planet of the Apes”, where Taylor realizes he’s back on Earth and condemns the folly of man by screaming out, “You maniacs! You blew it up! KLAATU BARADA NIKTO!!!”
I’m amazed at how many people seem to misremember this. The actual quote is from the end of Chinatown, just after Faye Dunaway gets shot:
“Forget it, Jake. Klaatu Barada Nikto.”
Wasn’t that the theme song Julie Andrews sang while running through the hills of Austria?
“The hills are alive…with Klaatu Baradu Nikto…(La la la la…)”
…I thought it was from a different Charlton Heston movie:
“Soylent Green is made of KLAATU BARADA NIKTO!!!”
And how can we forget the Clarke Gable classic…
“Frankly my dear, Klaatu Barada Nikto.”
To which Vivian Leigh replies…
“Tomorrow is anoher Klaatu.”
Patty