Well, I have not expected to see this "that fast" - but it was released this week:
Microsoft IIS 7 WebDAV 7.5 / FTP 7.5 Extension
This should solve the issues with Apple OS X and the other WebDAV Clients like Subversion, Git, etc. which need locks. I will give it a try later this week and see if it fulfills the expectations.
Here's an article on IIS.NET which gives a detailed How-To about the Locking Configuration.
