Former stripper Channing Tatum gets back to his ab-tastic roots in tomorrow's Magic Mike, complete with sexy dance moves and a bevy of stripper-approved costumes. But Channing's first onstage experience involved a not-so-hot stripper ensemble — a Boy Scouts uniform — which he called the most embarrassing moment of his life on the Late Show With David Letterman last night. It was bad enough that he had to don the kiddie camper outfit, but Channing recounts that to add insult to injury, he also had to sing during the bit . . . with his sister and her friends in the front row. Watch him bare all to Letterman now!