Strange, i managed to set up syncing for two laptops with the desktop app. It works fine. But sadly not for the iPhone... iPhone just receives the updates, but it doesn't manage to acutally update the DB via WebDAV, it just creates more of those:
OmniFocus.ofocus-copy-in-progress-(randonStuff)
MichaelGs way (below) didn't work for me.
http://forums.omnigroup.com/showthread.php?t=8615