One of the most lauded features in ASP.NET 2.0 is Master Pages. Master Pages serve as templates for a site, making it easier to create and maintain consistent style site-wide. They do have one drawback under VS 2005: they cannot be nested.
Nested master pages are essential for larger sites that have more than one layout. Without them, developers have to manually ensure the common elements in each master page such as banners and copyright notices are kept in sync.
It turns out that nested master pages are actually supported by ASP.NET 2.0 itself. It is only in the Visual Studio 2005 IDE where they cannot be used. Visual Studio 2008 adds full support for nested master pages, allowing developers to switch to this model without losing IDE support.
It should be noted that since this feature is in ASP.NET 2.0, developers will not need to deploy .NET 3.5 to their production servers. They merely need to ensure that the project is set to compile to .NET 2.0.
Scott Guthrie has a walk-through of nested master pages on his blog.