Many Mac OS X users have reported a number of compatiability problems between the built in OS X WebDAV client and our WebDAV server. We'll be looking into these over the next couple of weeks and will let people know when we have ironed out the issues. For now, it's probably best to avoid using WebDAV with Mac OS X and just use the web interface for now to upload/download files.
Update: I believe it's fixed now. The issue was with Mac OS X insisting on creating locks on files, and our server not supporting them. I have implemented locks, and have positive responses from users on the forum. Yay.
There are still speed issues with Mac OS X, which we can't solve - it makes very inefficient use of the DAV protocol with what appears to be a complex temporary file creation mechanism designed for local disks. It makes many tens of requests where a more efficient client could create a single request asking for all the information it wanted at once.