According to:
http://haacked.com/archive/2010/05/16/three-hidden-extensibility-gems-in-asp-net-4.aspx
This new attribute allows you to have
code run way early in the ASP.NET
pipeline as an application starts up.
I mean way early, even before
Application_Start. This happens to
also be before code in your App_code
folder (assuming you have any code in
there) has been compiled. To use this
attribute, create a class library and
add this attribute as an assembly
level attribute. A common place to add
this would be in the AssemblyInfo.cs
class within the Properties folder.