While browsing on the MSDN website, the following article caught my eye: Replacing the DHTML Editing Control in Windows Vista and Beyond. It seems that starting with Windows Vista the fast and dirty way to add WYSIWYG editing to your web pages with IE won’t be available any more because the needed ActiveX component won’t be included in the distribution. It will still be downloadable optionally however. There are two migration paths for all of you using this feature (for either in-house or public sites): the MSHTML editor or one of the full featured cross-browser compatible alternatives.
Update: the guys over at the boagworld forum pointed out two more editors: DevEdit and Dojokit.