As of yet, they do not decorate the HM in WDW. Even though someday it would be a nice treat for those of us that don't make it to DL (too long of a flight for me).
Was at Disneyland last October during which they did the "holiday makeover" at the Haunted Mansion (they were quick - seemed to happen overnight) under the tag line of "When Two Holidays Collide" (Halloween and Christmas). Lots of pumpkins and "Zero" from Nightmare Before Christmas was featured.
At this time, only the Disneyland and TDL Haunted Mansions get the Nightmare Before Christmas overlay. Disneyland is in its fifth year, and TDL is in its second year.
The DL mansion closed Monday (9/12) for the installation, and will reopen on September 30th. The Mansion usually closed the first weekend after New Year's for removal of the overlay. Sorry, but I don't know TDL's schedule.
The general concensus at the Doombuggies message board is that the WDW Mansion will never get the overlay (for a number of reasons), but that's just opinion. In any event, WDW is not getting the holiday overlay this year.