The arrival of this month's MSDN Magazine was greeted with more enthusiasm in my wife than normal: "Your name's on both sides this time!".
(The other side was my address for delivery purposes :).
My article is available online here: Silverlight Security: Securing Your Silverlight Applications
It isn't a tour-de-force of in-depth hardcore technical security topics. Instead the aim was to pull together some of the many concepts that are often encountered when creating a Silverlight (or indeed, any RIA) application.
Since the article was published I came across a similar (though thankfully not too overlapping) security paper from Nick Kramer
who works on the silverlight team. Here's his blog post introducing the paper: Silverlight security overview paper has been officially published
. It gives a lot more detail on client side security (and reasons decisions where made by the Silverlight team) whereas mine focusses primarily on the Services your application would consume.