NEW YORK, Jan 29 - The number of help-wanted ads in U.S. newspapers fell slightly in December, suggesting some nervousness about hiring on the part of American businesses. The Conference Board said that its employment barometer dipped to 38 in December from 39 in the prior month. One year ago, the index stood at 39. "Business executives continue to express caution about investing, as well as new hiring," said Conference Board economist Ken Goldstein. "This is the hurdle the building economic momentum has to jump over," he said. The Conference Board compiles its index by surveying help-wanted ad volume at 51 newspapers across the country each month.///