If a condo developer is unable to sell enough units quickly enough to cover their construction costs, they will often convert the unsold apartments (or the entire development) into rental units. For example, The new development at 375 Myrtle Avenue in Fort Greene sold very few units so it converted to a rental. Go here for more info --> http://www.brownstoner.com/brownstoner/archives/2008/10/the_
if he cant sell then the condo developer can rent them out until he market improves. This will be common place in overbuilt area like Williamsburg, Brooklyn. The developers specing on the $1000 a sq/ft "luxury" construction and now proud owners of "luxury" rental units.