October weather can really vary. Check the forecast a few days before you leave, and adjust your packing from there. We always go in October, and have had:
1) Extremely hot and humid
2) Quite hot and humid
3) Hot and not quite as humid
We ended up with a huge temperature swing in October 2005, with Hurricane Wilma. Two days before Wilma: very hot and humid. Wilma swept through, then the five days afterward were very pleasant, with very little humidity (it was even too cool to sit out by the pool!). You just never know.
I always bring one long-sleeved item that I can carry with me (this year, it will be a long-sleeved white knit cardigan). I can toss it in my backpack and put it on if I end up in really cold air conditioning. I've been really glad to have it many times. I also have one pair of long pants (which I wear on the plane, since I'm coming from a chilly area in October). Have always been glad to have one pair, just in case. That's the hardest thing to buy at WDW, clothing-wise -- you can always buy t-shirts and sweatshirts, but it's hard to find long pants if you need them!