[cincyfan]

Okay, I fixed my own problem, so now I'm feeling foolish for not doing more research before posting. I'm going to leave this post here for those who follow.

1. Turn off Anonymous access in Directory Security and the Sync tab in Synchronization Preferences will ask for a username and password. You get to decide if you want integrated Windows authentication, Basic or Digest.

2. Make sure to add a custom MIME type for the folder for .client:text/xml on the HTTP Headers tab | MIME Types dialog.

3. Set the folder to allow directory listing, write and read on the Directory tab.

You should be good to go. If enough people are interested, I could offer hosting for a nominal fee. I've got the servers and the bandwidth. Hopefully I've given enough information so that a reasonably savvy Windows Server 2003 admin could set up WebDAV for themselves, though.