Charlize Theron has cause to celebrate: the South African starlet has just become a U.S. citizen, she revealed on the Late Show With David Letterman last night. "I've always wanted to be one; they just didn't want to take me," she joked. The Academy Award-winning star has called the U.S. home for years, but now it's official: "You're stuck with me now!" she raved. It took her some serious work. "It's quite a process - you have to work hard, you have to study up. You have to know your stuff; you've got to know your senators... They give you a test." Thankfully, she passed with flying colors.