research for the blog post published yearly with our Christmas and New Year's Eve recommendations has been almost completed. As always in Rome it's not easy to get in advance information about openings and menus. Thank you Gillian for your kind mention.
On Christmas day if you are looking for great Roman food Flavio (in Testaccio) and Cesare al Casaletto (not in central Rome but quite easy to reach with 8 tram from Largo Argentina) will be open with a bit narrowed menu choice than usual.
For a high-end restaurant I'd definitely recommend Pipero al Rex. They will offer special tasting menus: an amazing Christmas day menu at 150 euro per person and 10 "surprise" dishes at 180 euro on Christmas eve.
I'd add Imàgo at Hassler Hotel too. They will offer a Christmas brunch at 130 euro, but I can't comment about it since I've never experienced their buffet offers earlier.
The 26th is a public holiday too and not all venues and places will be open, but I'm sure you'll find enough choices.
Enjoy Rome during Christmas Season!
Having spent Christmas week in Rome in 2009 I can attest to the fact that Rome IS NOT "completely closed" on Christmas Day. As we get closer to the date, check out Katie Parla's web site for restaurants that will be open as she always posts a list and her suggestions are spot on