We leave in 3 days for our 5th mid-late December trip in last 6 years. In that time we have been there for recorded setting days on both ends. In 2010 the morning we left it was -4 C, in 2011 we had a heat wave almost the whole trip that saw a Dec record temp one day of 33 C. In 4 trips we have spent a total of 51 days in Orlando in mid-late Dec, I guestimate that about 41 of those days had a daytime high of 20 C or higher. The majority of the 10 days that were cooler came all in one trip in 2010.
So the odds are in favor of mostly nice weather, but you really need to prepare for everything from 0 C to 30 C LOL!!