We just returned from 5 days at WDW over New Year's. We wore pants and short-sleeved shirts during the day, although it ended up being warm enough that we could have easily worn shorts instead. However, I would definitely recommend bringing a lightweight jacket or sweater. I tied a thin cotton jacket around my waist every day, since we often didn't go back to our hotel before dinner. By the time the sun goes down (and you are in an air-conditioned restaurant), you'll definitely want something to cover your arms.