Silverlight is designed to make the update experience as seamless as possible, but its designers also considered security awareness as well as enabling user or enterprise managed overrides.
Silverlight knows to kinds of updates .
1. Security related updates. Silverlight checks for security related updates once every 24 hours. Security related updates patch vulnerabilities, but do not provide new features. The 24 hour check interval minimize your exposure to known vulnerabilities.
2. Feature updates. Silverlight checks for feature updates once every 30 days. Feature updates are releases delivering new capabilities, you know the things that help you build even better Silverlight experiences. For example, Silverlight RC is going to check for the 1.0 release once every 30 days.
The Silverlight control has to be running for those checks to occur -- which means the control has to be playing content (which shouldn't be a big deal since there's going to be a LOT of content). When Silverlight will automatically downloads the new version in the background when an update is available and then runs the installation in silent mode to make the new version available the next time your browser starts. Unfortunately, the restart is necessary to unlock the loaded DLL -- remember the control had to be loaded to check for the upgrade. Silverlight will not prompt you to restart the browser if it downloaded a feature updates though. The new version will just magically be there.