Unfortunately what Valentina writes is true - last April the city's artistic preservation board had announced the Corridor would be closed for restoration by the end of 2009. The corridor was opened for "extraordinary" periods from May through June and then from the end of September to December 18, 2009.
Since no additional "extraordinary" openings have been announced, we'll have to assume that works indeed have started or about to start.
The sad thing is that the Corridor is not slated to be reopened until 2013! At that point it should have new systems of air filtration, security, lighting and other work completed and it should also be accessible to use as a connecting point between the Uffizi Gallery and the Pitti Palace as it used to be at the times of the Medici. Should be neat but I guess we'll have to wait a while...
I'd like to inform anyone interested in visiting the Vasari Corridor that it will be open from February 2 to April 15th 2011. You can find more informations about the guided visits in this dedicated blog post. Don't miss the chance to visit this special place!