We were in Orlando from December 31, 2011 - January 7th, 2012. It was our second January visit.....we were also there the second week of January in 2009.
This year, it was beautiful when we arrived. New Years Day was fantastic, with sunshine and highs in the upper 70's (we were at Discovery Cove and couldn't have wished for a nicer day.....warm, sunny, and clear). Shorts and t's were perfect.
By the 3rd and 4th of January, we were decked out in winter coats, hoodies, jeans and gloves and could literally see our breath in the early morning hours at Universal Studios (I literally had to go into the women's washroom at the Wizarding World of Harry Potter while the boys were riding Forbidden Journey because I had almost lost feeling in my fingers

). The news was all abuzz about the "Florida Freeze" and the poor locals were just about dying

.
By the 5th, we were back to being comfy in jean's and t's with a sweater early on, and by the 6th, it was hot again.....shorts and t's and glorious.
Be ready for just about any weather. The great thing in it all......out of two week long January visits, we have only experience rain once (for an afternoon in 2009). The rest of the time its been sunny, blue skies and absolutely fantastic (even when it was cold). All the pools are heated, so us hearty Canucks can swim regardless of the air temps.