The IHE has had a profile for being able to test interoperability of healthcare provider directories. Just recently, IHE USA has started their own version, which should be compatible with the international IHE version, in addition to providing extra details to accommodate for US needs.
An HPD is just what it sounds: it's a public directory of private health providers or of institution health providers. The goal is to have a standard for this directory, such that every group, individual, organization or institution who is maintaining one, could have the ability to share it, and make a more comprehensive and up to date version available.
For example, with a standardized protocol for the healthcare provider directory, there could be organizations which maintain a central directory by providing a simple user interface to download or update the directory, allowing other institutions to make use of the latest and most up to date version available.