Three weeks ago Joe Duffy published an article about 'Monitor.Enter, thread aborts, and orphaned locks' on his blog. After reading this article and responding to it (see my comments), I was pretty much in shock. I came to the conclusion that the C# ‘using’ statement isn't 100% safe.
This article describes how to inherit your .aspx pages from a custom class, in such a way that you can use nested master pages, without getting that terrible 'Design view does not support creating or editing nested master pages. To create or edit nested master pages, use Source view' from the Visual Studio Designer.