Windows Mobile Device Center (WMDC) can be found on the control panel in Vista RC1. Unlike its similarly sounding Sync Center this application is essentially, from my perspective, a fancy looking active sync with more features.
Interestingly enough the Sync Center once the WMDC is installed will include a category called 'Devices' and displays the current status. So one can think of WMDC as a configuration tool and the Sync Center and an information viewer (incidentally a right click on the device in sync Center provides a link to the WMDC.
I don't believe it can be generally activated for the public release (as the team is diligently working on some key feature enhancements as we speak). I had to use an internal tool to poke Windows Update to download the application. Without it WMDC appears only with configuration options but does not fire up an interface.
The new WMDC application is pretty slick allow a quick view of what's on the device as well as more configurable options than previous active sync. It also would appear to be device aware (i.e. recognizing the differences between Smartphone and pocket pc). Currently WM5 is supported.