The developer preview of Windows 8 has a lot of developers starting to think ahead about what kinds of applications they will be writing. While Web apps are more work to write than desktop apps for certain functions, they are cross-platform and look good on Windows 8. Traditional apps (now considered legacy), at this time, are a poor experience but are backwards compatible. The new Metro apps look slick, but it is difficult to do sophisticated UIs with them, and they lack backwards compatibility.
Right now, I think developers should seriously consider abandoning the desktop model in favor of Web apps if they can, but I know that not everyone can do that.
Will you write Web, legacy, or Metro apps for Windows 8?