I just noticed an interesting product mentioned on the WebDAV.org site:
HiPerExchange is a WebDAV server that runs on the machines of individual email users, allowing them to efficiently access email using Outlook Web Access. See the HiPerExchange Technology Primer for more detailed information.
The WebDAV server runs on every client machine and essentially acts as a local cache of the data accessible to the user on the Exchange server, both private and public folders. As I mentioned here Outlook Web Access uses WebDAV so the user simply points their browser at their local HiPerExchange server instead of the Exchange server. Sounds like a cool project to work on.