City Comptroller William Thompson released a report criticizing the city's housing authority for leaving thousands of city-owned apartment vacant for too long. On average, 2107 apartments were left empty for 40 months while the Housing Authority would renovate the apartments, and Comptroller Thompson pointed out that the city could have made $4 million more in rental income if the apartments were vacant for an average of 32 months.
The Housing Authority has 140,000 families on the waiting list for city housing and says that the long vacancies are partially due to the extent of the renovations (and dealing with bids along government guidelines). We're not sure whether we should be shocked at the length of the vacancies or consider them typical for the city (government isn't exactly Extreme Makeover: Home Edition). At any rate, residents don't like the long-vacant apartments because they get broken into, with drug dealers taking them over.
Here's the NYC's Housing Authority's capital report for 2006 - there are 2,691 residential buildings and over 180,000 apartments in the system.