The Edith Piaf song, "Non, je ne Regrette Rien" is used by characters "Inception" like an alarm to wake from dreaming. It's a lovely touch, and one exploited by composer Hans Zimmer in assembling the film's entire score. Here's a handy audio comparison [I saw on Cinematical] to cut straight to the big reveal:

Technically, Zimmer didn't just slow the Piaf song down and call it a day, but extracted bits and then used his electroturdmatics to reconstruct a theme in varying "subdivisions and multiplications of the tempo of the Édith Piaf track," he told Dave Itzkoff writing for Arts Beat. Normally I'd be restraining myself from using foul language to discuss Zimmer's approach to creating film scores, but credit where it's due. I love the idea and the use of Piaf to achieve the encompassing theme. Zimmer is clearly still giddy over the whole thing. "So I could slip into half-time; I could slip into a third of a time," he said, tripping his balls off over the tempo manipulations he employed to great effect. "Anything could go anywhere. At any moment I could drop into a different level of time."

A question remains whether or not he will be eligible for Oscar recognition, given the Academy's strict rules -- is it Hans Zimmer or Edith Piaf we should be awarding? Zimmer first said he only extracted "these two notes out of a recording," but later stated, "I didn't use the song; I only used one note." Perhaps both are true, depending on which level of time he happens to be dropping through. I think he's at least one step ahead of the Academy.

More on "Inception" score.