The Vasari Corridor, while scheduled to be renovated, has been open the last couple of years on a limited schedule. Publicly available tickets are nearly impossible to come by however, leaving just about the only chance of visiting this historical walkway to private, guided tours.
One such tour is run by a great local Florentine company: florencetown.
They currently offer guided tours (until at least the end of June, 2010 according to their website) every Tuesday and Friday at 3:30 PM.
For more information, availability, or to make an inquiry, visit them at: florencetown.com (you can also find many other tours and activities for Florence and Tuscany on their website).