Ahhh thats a big one...

What is supported:
  • Non-recurring appointments
  • Contacts in Microsoft Outlook that you create or modify 
  • Folders that you create in Outlook and in Exchange 2000 Server or Exchange Server 2003
  • E-mail messages that you create or modify

"All other tasks that use the WebDAV protocol to modify a Web store in Exchange 2000 Server or in Exchange Server 2003 are not supported."
 
Here is a support link:

  Available support for applications that use the WebDAV protocol to access Exchange 2000 Server or Exchange Server 2003
  http://support.microsoft.com/kb/920134


To be more specific, here are a few which are not supported:

  • Tasks - you may be able to create some basic ones which works in many situations, however may not work in all.

    Its best to use a supported API for this.

  • Anything with Meetings, recurring appointments - basicly anything beyond reading a basic appointment.

  • Journals

  • Notes

  • Working with RTF Text

  • VCALENDAR/iCAL messages.

There are some properties which WebDAV just cannot get at to have some of these work properly.  Also, there are is no documentation covering these for WebDAV. Another issue with WebDAV is that its all stick and bricks - ie there is no business logic and thats whats really needed for things like doing meetings, recurring appointments, cancelations, working with timezone information, etc. Using WebDAV to do these things is like taking a trip down a long dead-end road - it seems like your getting somewhere and you have success and then it happens - you reach the end and there is no way to go forward...  then you realize that you can only go back and start over - with a different API.
 

Where WebDAV fails to deliver in the areas of calendaring, Exchange Web Services (EWS) rocks.