LittleBigPlanet Karting is accepting beta applications right this very moment. If you're interested in a PS3 Sackboy-Mario Kart-style mash-up, sign up through the PSN Beta page.
Sony of course warns that not everyone will qualify for the beta, but we assume the question "Do you own a PS3?" may be one with a correct answer, while "Which of the following games have you played before?" may be more lenient.