Yesterday night I had decided, that this morning I was going to sit down and write a post on the uses of the IsSectionDefined method in the Razor syntax, and how useful it was for determining if views were implementing certain sections in a layout. (which is very useful and provides a great deal of flexibility in your design)
However this morning I awoke to find that Scott Guthrie had written about the same subject, and who am I to compete against The Gu.
http://weblogs.asp.net/scottgu/archive/2010/12/30/asp-net-mvc-3-layouts-and-sections-with-razor.aspx
Enjoy!