The National Association of Home Builders (NAHB) says there is pent-up demand for apartment rental units.  Although there is new construction, the ongoing credit crunch is making it difficult for developers to meet the demand, which could lead to higher rents.  The report cites the Multifamily Vacancy Index (MVI), which is at its lowest level since the third quarter of 2006, indicating fewer vacancies.  Another index that measures builders sentiment about new construction is at its highest number in four years.  The report says certain markets area already seeing an increase in rents.