Right, so I was on the PCT this summer. Mostly the desert section was as you'd expect it: plenty warm. The colder weather generally struck in the mountain ranges, and only briefly: We had one windy/cold night just past Mt. Laguna and one VERY windy/cold night in the San Gabriels, near Wrightwood. I was carrying a silkweight short-sleeved shirt, a silkweight long-sleeved shirt, a Thermawrap vest and a windshirt. I was fine except for my hands, which were too cold to set up my shelter properly. Lots of flapping that night.
Last year we saw snow before the Sierra, but not much, and nothing like you'll see after KM. All that said, conditions largely depend on the year. I heard in '05 there was a ton of snow around Mt. Baden-Powell, but in '06 it had mostly melted.
One more thing: Don't believe the hype! I wouldn't skip an allegedly "snowy" section until I had seen it myself. There're a lot of rumors on the trail.