Expect temps between 30° and 80°, potentially covering the entire range in a week's time.
Winter in FL is very unpredictable. Averages paint a very nice picture - low to mid 70s during the day, low 50s overnight - and when it is average to warm it is fabulous. But every winter seems to bring one real cold snap with lows at/near freezing, and there's no way of predicting when that will come. We come from a cold climate and don't mind the unpredictability too much because even FL cold isn't cold when you've just left a Michigan winter and the low crowds more than make up for the less than ideal weather, but it isn't a time of year to go if a cold snap will put a damper on your vacation.
That was our experience when we went in January 2010. We saw record-breaking cold that week. Some days the high temperature didn't even hit 40°F. We kept our gloves and heavy coats on all day some days. It was actually colder in Central Florida that week than it was in Northern New Jersey where we live. Then the last day we were there, highs were in the 70s and I was so sad to leave.
I expect that ridiculous cold snap to come late this year, in early March, and I'll tell you why...
We went to Disney in mid-late Jan in 2008. Had a great time, lovely daytime weather with a few cool nights and a couple of days that were plenty warm enough for swimming and waterparks. Those same dates in 2009 were record setting ridiculously cold and CNN was talking about all the crop damage.
In 2009 we went in mid December. Again, lovely weather - near-record highs for the first couple days with cool nights later in the week, and some nice swimming and waterpark weather on several days. The same week the following year brought a ridiculous cold snap and threads on the boards about how all Orlando was sold out of gloves and hats.
Our last trip was the first week of March '11, so anyone going that time of year in 2012 might want to pack some cool-weather clothes just in case!