From Microsoft's MSDN site: http://msdn.microsoft.com/windowsvista/default.aspx?pull=/library/en-us/dnlong/html/AppComp.asp

Deprecated Components

The following components from earlier Windows releases will not be present in Windows Vista:

• WinHelp and HTML Help (.chm) files support: Windows Vista introduces a new help model called Windows Help. All applications need to update their help documentation to the new model for Windows Vista versions. Older formats will continue to be supported for legacy applications through a downloadable WinHelp component, which will not be in Windows Vista by default.

Given the pain of switching formats and the difficulty of policing proper downloading/installing of additional components by end users (we've been through this before!!), I'm afraid Microsoft is hastening the end of proprietary client-side Help formats rather than encouraging software producers like us to keep up them. One more driver towards web-based Help.