Nesting Master Pages, Do It At Your Risk.
I love Master Pages, it was one of the two main reasons I can dump DotNetNuke in favor of ASP.NET 2.0 alone. The other being Membership. I hvae found using master pages to be quite easy and extremely useful. I won't get into what I used to do here, but it was a similar idea, just not nearly as clean.
I use Professional ASP.NET 2.0 everyday now to help me get the most out of ASP.NET 2.0 and Visual Studio 2005. Starting on page 272 there is a section on Nesting Master Pages. This is a great concept because now you can create a core layout and then create sublayouts and have everything stay consistent. One big problem though, you can no longer work in design mode :(.
This sucks, this is the same problem Visual Studio 2003 has with inheriting base classes for your page, you loose design mode and thus a lot of time in developing your site. The authors do mention this sad fact at the end of the section. I just wish the guys at Microsoft would have helped us out on this one.
Anyway, I still love Master Pages.