notify client from server

If I have an asp.net application, and I want to notify the user when something changes on the server, how do I do this.
The same thing with a silverlight client. For the silverlight client is there some form of subscribing to an event on the server from the silverlight client?

Is there a way to insert Silverlight XAML into my page using ASP.NET code-behind?

So, I want to create some Silverlight charts from some data on my ASP.NET web page. The way to do this, either using Visifire (my current thing) or the upcoming Silverlight charting, seems to be to create some XAML, or at least some Silverlight objects (like in this Visifire sample), and then stuff them into the page somehow.