ScottGu summarizes the new web designer and web development features in VS 2008. It uses the same HTML designer as MS Expression does and HTML and CSS support is far broadest as never before. The cool thing is that you can use these new designer features on any ASP.NET 2.0 also because of the new multi-targeting support.
If you use ASP.NET AJAX, the new version for .NET Framework 3.5 includes many improvements.
And finally – there is nested master pages support.