Microsoft just released an update to Web Folders. This update apply to Windows 2000, XP, 2003 and Vista.
They say it corrects the problem describe in kb892211
This fix does not seems to do anything for those of you who got the '401, Unauthorized' error on the root dir and the /dav dir when accessing a webDAV folder on WebCT (Linux).
I'm not sure yet if this update modify the WebDav Mini-redirector of Vista or only the WebFolders package from Office.
Download it : www.microsoft.com/downloads
May, 25th update
It is (kb907306) an update to the WebFolders and not the Mini-redirector (WebDAV). And I was partly wrong, it does fix the '401, Unauthorized' error by replacing de mini-redirector by the WebFolders package. Although I'm now able to connect to my webDav folder on WebCT, I can't modify any file (save).