Loosely translated, Italian for "Whatever shall be shall be." Literally translated (by babelfish) "That it will be will be."

Quoted often over time, I'm sure, the only time it has been used that I can think of is in Marlowe's Doctor Faustus.
From scene 1:

The reward of sin is death? That's hard.
Si pecasse negamus, fallimur, et nulla est in nobis veritas.
If we say that we have no sin,
And so consequently die.
Ay, we must die an everlasting death.
What doctrine call you this? Che sarà , sarà:
What will be, shall be! Divinity, adieu!
I believe there's also a song from a Disney-ish movie that sings these lyrics, but I may just be imagining this in my head.
Legbagede /msg'd me: "Doris Day sang it in Alfred Hitchcock's The Man Who Knew Too Much - If that's what you're thinking of." Quite possibly, but who knows what's going on in my mind...
Sylvar also /msg'd me: "About Che sarà sarà: Doris Day sang the Spanish equivalent, Que sera sera; you can hear it at the beginning of the movie Heathers.
And Mink has a w/u saying the same below, but in more detail :)