Michael Douglas and current squeeze Catherine Zeta-Jones have sent out invitations to friends. The lovers say their party, to be held on Sept. 25, will be a joint birthday bash for the two of them. (He'll be 55, she'll be 30.) Some others are calling it an engagement party. Douglas's publicist told the New York Post that although Michael and Catherine have no concrete plans to tie the knot, he would not entirely rule out the possibility. "September's a long way away," said Allen Bury.