I am using WebDAV in conjunction with SharePoint portal version 22.214.171.12424.
I also testing using WebDAV via the built-in provider in IIS on Windows Server 2008.
The thing that I found is that GetFile or GetFileSystemObject for a file does not work with SharePoint.
I debugged the problem and SharpBox is issuing a PROPFIND with a Depth=1. That works fine when using IIS but not SharePoint which returns a 404.
FWIW, if I change the Depth to 0 in Fiddler and reissue the request then it works.
My plan is to work around the issue by always walking from the folder to file.
Just an FYI in case someone else encounters this.