Flannel-loving starlet Mary-Kate Olsen was in no condition to be on David Letterman's Late Show last night. Her excuse for her disjointed, unenthusiastic interview? She was "so tired" after her long trip (on an airplane, of course, not via any illegal drugs or booze or whatnot). Letterman could hardly have sounded less impressed with this, and twice referred back to how sorry he was about ...