I was investigating access to the root folder of Exchange mailboxes yesterday and came across am Exchange-oriented WebDAV explorer called Mistaya and a .NET WebDAV API called WebDAVlayer, by Henning Krause. These may be of use for anyone doing work with the WebDAV functionality of Exchange.
It looks like you cannot access the root folder via WebDAV, only the "Top of Information Store folder" sub-folder which corresponds to the top level of the view you see in Outlook, the folder containing Inbox. I'm working on an application which stores information in an item in the root folder and given all the trouble we have had with MAPI - threads hanging if objects are released when the Exchange server goes offline, very long calls at random intervals which can't be timed out, etc - I was interested to see if we could use WebDAV instead. The big advantage of WebDAV is that we would have complete control of the client-side code, instead of relying on the code in MAPI32.dll. This would make it easy to implement timeouts on calls. Ignoring the lack of root folder access for the time being, the next step will be to compare the performance of the two protocols.