Any recommendations for a December trip for cage diving with Great White Sharks out of the Cairns area?

Great White Sharks are not found in waters off Cairns, to go cage diving with them I think you'd need to go to much colder waters of South Australia.

Pat's right,
The furthest north in Queensland that Great Whites are common is around Hervey Bay /Fraser Island especially around the time that whales are giving birth.
The sharks off Cairns are generally relatively timid reef sharks. Beyond the reef in the Coral Sea there are all manner of big toothed sharks like Tiger Sharks, Whalers and Hammerheads - and any number of Dive operators will take you , but it won't be in a cage, and the trips will take several days !

Your best bet for a Great White cage dive is South Australia with an operator like Rodney Fox. Try this URL
http://www.mikeball.com/great_white.html

Alternatively if you just want to do a one day shark adventure, a lot of aquariums will let you dive with harmless Nurse Sharks that look big and scary. Try SeaWorld SharkBay on the Gold Coast or one of the Sydney aquariums
http://sharkdive.oceanworld.com.au/program.asp

Good Luck
Mike

If a Great White Pointer comes any further north than Victoria's coast line, it's bloody well lost, or in the middle of a long distance hunt of something delicious.

The only Great White Pointers you see in Sydney and northwards are the norks attached to topless women bathers.