A. Server 2008 R2 introduces a complete set of PowerShell cmdlets for AD that rely on the AD Web Service (ADWS) for communication with AD. This ADWS is installed by default on all Server 2008 R2 domain controllers (DCs). The new ADAC provides a management interface for basic object management and utilizes the PowerShell cmdlets, which means for the ADAC to work, the DCs also need the ADWS.
Microsoft has released an update for Server 2008 and Server 2003 DCs to let them provide the ADWS, enabling PowerShell and ADAC management, through the AD Management Gateway Service.
Note that while this update enables ADWS on those DCs, you still can't install the PowerShell AD cmdlets nor the ADAC locally on non-Server 2008 R2 boxes. You will, however, be able to remotely manage them.