SORRY, ladies, but Usher is officially off the block. Sources say the R&B icon is engaged to his longtime girlfriend and stylist Tameka Foster. "It's 100 percent official," said an insider. The couple began dating more than a year ago, but tried to keep it quiet - going so far as denying their relationship - but they recently became more open with their love. "She is wearing a beautiful ring," said a friend of Usher, "and he's been introducing her as his fiancée." A rep for Usher declined comment.