Windows Media Player 10 (WMP) introduced the User-mode Driver Framework (UMDF). UMDF allowed drivers to be run in user-mode and the WPD team was the very first client. The WPD team delivered the MTP (Media Transfer Protocol) driver along with a Service Provider for WMDM (Windows Media Devices Manager) used by WMP. This driver stack allowed portable media players (supporting MTP) to be installed as first-class Windows devices.
With WMP11 and Windows Vista, the WPD team has widened its horizons and will be delivering a brand-new set of WPD APIs. You can use these APIs to talk to a whole range of portable devices (think cellphones, cameras, media-players, GPS units, etc.). There's a brand new WPD driver framework as well.
The WPD API and DDI will be part of the Vista Platform SDK and anyone can build their own WPD application or their WPD driver. It's really easy :).