I hiked south on the PCT through the JMT area from about Sept 15 to Oct 1 last year. There were a couple of early storms, as Shelly said. However, as Kanga mentioned, the snow from these storms (the ones that I saw, at least) did melt out in a few days.
I was at Crabtree Meadows around September 27 last year. We got 4-5 inches of snow there, and other hikers who attempted to summit Whitney that day and the next turned around because of deeper snow above Guitar Lake.
But other hikers that I know DID summit Whitney later in the season (in the Oct 3-9 range, with no special equipment), and seemed to have no problem with it. I went up Whitney from the Whitney Portal side in early October, and the trail down to Guitar Lake looked perfectly fine. Two hikers that I know of summitted Whitney from the Guitar Lake side that day with no trouble.
So, it just depends. If I were to hike in the Sierras in mid-October, I would be prepared to get snowed on. I would also prepare for the possibility that the snow might slow me down considerably or prevent me from (or at least, delay) going up Whitney.
2005 PCT Hiker