Hi Fuzzy, I have done a number of winter trips in the canyon, and do not own a dry-suit, so you should know there are many ways to skin the winter cat. Bib bottoms and a good splash jacket or rain jacket with lots of poly pro can serve you just fine, as they do me when i row. It depends if you are a passenger just sitting, rowing your own boat or kayaking.
Go Man Go! You will have a blast! Yes, it's cold, yes, it might and has snowed on the river in December. It will most likely freeze at night, but not drop much below freezing. The river doesn't freeze over, but I have seen a lot of ice built up on the shore on a couple of trips.
There's a huge wealth on info at these two sources:
River Runners For Wilderness’s listserver. It's a great place to stay abreast of and participate in the latest river issues. It’s as easy as sending a blank e-mail to Rafting_Grand_Canyonfirstname.lastname@example.org.
Also, check out RRFW’s Rafting Grand Canyon Wiki for free information on DIY Grand Canyon rafting info http://www.rrfw.org/RaftingGrandCanyon/Main_Page including info on winter trips.
Finally, e-mail me direct if you have further questions. Have a wonderful journey, Tom Martin