According to Microsoft’s Silverlight 4 Privacy Statement,
Disabling Silverlight Unique Client Identification
A registry key HKLM\Software\Microsoft\Silverlight can be created with the Value Name EnableClientInformation with a Value Type of REG_DWORD and given a value of 0×00000000. This sets the machine to never provide the unique identifier associated with the Unique Client Identification feature to any Web site regardless of the DRM setting.
Any downside to this?