Epic would be my pick for a nice, quiet place for a Christmas lunch. It's showing availability for the 12 noon time slot on Christmas Day, but doesn't seem to be showing availability for dinner. www.opentable.com
Avenue or Lai Wah Heen would be my next picks, out of the "nicer" (as in more upscale) restaurants that are open.
from www.opentable.com, for 7 pm on Christmas Day.
Annona At The Park
Avenue Bar and Lounge
Cafe Nicole - Novotel Toronto
Dazzling Modern Restaurant & Bar
Lai Wah Heen
The Marquis Lounge & Bistro at the Hilton Garden Inn Toronto
Signatures Restaurant - Intercontinental Hotel Yorkville
Studio Cafe Restaurant @ Four Seasons Hotel
Toca by Tom Brodi- Ritz Carlton
plus most of the restaurant's listed on Toronto.com's 2010 list, apart from Bright Pearl and Lai Toh Heen which have closed, and Cha Liu, which has closed until it re-opens at a new location.
Lai Wah Heen
108 Chestnut St, Toronto, ON M5G 1R3, CA
100 Front St W, Toronto, ON M5J1E4, CA
Library Bar at the Royal York opens at 7 pm and takes reservations http://www.opentable.com/library-bar-the-royal-york-hotel?hpu=26732638&shpu=1&p=2&rid=47299&pt=100,100,100,100,100&t=fr&sd=12/25/2010%207:00%20PM&i=0&d=,12/25/2010%207:00:00%20PM,12/25/2010%207:15:00%20PM,,12/25/2010%207:30:00%20PM
Ciao Wine Bar in Yorkville is also open.
Here are some other establishments open on Xmas Day: