.NET offers so many choices for your presentation layer.
Windows Forms is mature, reliable, tried and true. You can use it to build smart clients like Outlook or Word.
ASP.NET gives you reach, and makes it easy to write rich, data-driven web sites.
Atlas promises to do for Web 2.0 what ASP.NET did to Web 1.0: provide just the right levels of abstraction and building blocks to make richer browser interfaces a reality.
Avalon (WPF) offers unprecedented power and potential for integrating 2D, 3D, animation and media -- for rich client and browser apps like this awesome North Face demo.
So... which should you be using?
The answer, of course, is "it depends!"
WinFX blogger Tim Sneath's recent blog entry, ASP.NET, Atlas, Windows Forms and WPF, offers answers to questions I get on these technologies all the time.
And Pieter, in the comments, directs us to presentation PRS200 from PDC05, called "Choosing the Right Presentation Technology: Windows Presentation Foundation ('Avalon'), Windows Forms, ASP.NET, IE, and More." You can watch the presentation online or download the Powerpoint slides.
(p.s. I haven't even mentioned some of the other presentation layer options, like Managed DirectX, VSTO, InfoPath, and non-.NET solutions like straight-up DirectX and Win32!)