It's that time again. time to indulge in a spot of jealousy, betrayal and late-night bar-hopping. Also, the moment of truth has finally arrived. Everyone's back from a week of après-ski hot tub fun in Sun Valley, and cream-coloured envelopes from the Ivy League are beginning to drop all over Fifth Avenue doorsteps. The question is: are the envelopes thick or thin? Has Nate's devotion to his misbehaving heiress dimmed his chances of getting into Brown? Is Blair too busy daydreaming in Eric's