When you install Silverlight, you'll find a small 90KB file called 'slup.exe' in the Silverlight application folder ("C:\Program Files\Microsoft Silverlight") , It is the Microsoft Silverlight Configuration Utility. I think its been provided since the newest version only.
This can also be obtained when you right click on any Silverlight app. to get 'Silverlight Configuration' option.
And the 'Updates' tab shows
As you can see Silverlight will now update automatically, that's the default .