In this post, Jeff Prosise discusses another new Silverlight 4 feature - notification windows, also known as “toast” windows.
This feature is available to all OOBs, regardless of whether they are trusted. It is not available to applications that run inside the browser.
A notification window is one that pops up in the lower right corner of the screen, similar to the ones that Outlook uses to notify users about incoming e-mails. (On the Macintosh, notification windows appear at the top of the screen rather than the bottom.) Notification windows are normally used to let the user know something important has happened.