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 0x00000000. 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?