For new participants,
see here for an explanation of Yuletide Madness!
Yuletide Madness will happen, although it's not yet certain what form the prompts will take, whether it will show all prompts, or unfilled prompts (probably the former). What is certain is that the prompts won't be up until sometime tomorrow, US time!
ETA: No, the actual Yuletide Madness 2010 collection is not yet open, however, nothing is stopping anyone from going ahead and writing!
Until the prompts are available, you can dig around in
the Dear Yuletide Authors post and see if any prompts there speak to you! And if you didn't link your letter in that post, or want to list your prompts there now, that's fine!
Note: On the AO3, the Yuletide Madness collection is for additional story treats that are under 1000 words (and only those under 1000 words!) What is certain is that the prompts won't be available until sometime tomorrow! Stories that end up over 1000 words will still be able to go into the main Yuletide collection.