This isn't the first time they collaborate. Back when Britney Spears released Circus, Ke$ha sang background vocals for a song called "Lace and Leather." The two are now back in the swing of things and Brit is ready to take over with her new single "Til The World Ends" from her highly anticipated album Femme Fatale scheduled to hit stores March 29. The next single, which should be out next week, will take fans into the mind of Spears, as she sings about a magical moment that she wants to last until the world ends. Sounds like a strong drug. The pop star is slated to have a track with Travis Barker on the album as well.