The TALK of fashion week in New York is not puff sleeves - it's the apparent coupling of Eva Mendes and Joaquin Phoenix and their fervent denials. Not long ago they finished the movie "We Own the Night " but can't seem to let go of each other. Despite the fact that they have been seen together numerous times looking QUITE affectionate (including last Thursday according to Rush & Molloy,) Eva flatly denies a romance. She insists she still has her live-in boyfriend - a handsome film maker named George Gargurevich, but they are rarely seen out together. We sense the makings of a love triangle.