Since some weeks Microsoft offers a Silverlight  update (KB946609 ) through Microsoft Update. As it is an optional component you have to select it in the Microsoft Update web site or with Windows Vista from the optional updates. As the knowledge base says it is an update it is working as new installation as well. I'm very happy about that because Silverlight gets on more client PCs.
Well, I have installed Silverlight right before it came to Microsoft Update directly from the Silverlight web site. Yesterday I have noticed on a colleague's PC that when installed using Microsoft Update you get a popup when visiting the first Web site using Silverlight. I didn't know this and only want to share this with you.