London Tube outages weekend of 26-27 November 2011
Also, all Tube lines will be in service during the December 12-13 and 19-20 weekends, except for Northern (Sunday outages), Waterloo&City (Sunday outages) and partial service on District Line on Dec 12 and 13 (full service on Dec 19-20 weekend).
Most Tube lines will have outages during all of Christmas week.
Complete outage info here through February 13, 2012 from Transport for London