[Lizard]

[I'm only posting this so you get an answer before Monday. I'd really like someone to post better/simpler instructions.]

I'm out of town at the moment, so I can't try this, but it ought to work:

1) Make sure all your OmniFocus data has been synced to one Mac.
2) Make sure you have a current backup of your OmniFocus data, and make a copy of your preferences file on each Mac just in case something goes wrong.*
3) Move, rename, or delete the OmniFocus data on your WebDAV server.
4) On each computer, open up com.omnigroup.OmniFocus.plist and delete the SyncClientIdentifier item.
5) Launch OmniFocus on each Mac and sync it, starting with the Mac that you're sure has all your data (see step 1).
6) If you get a "use client or server" dialog on any computer or iPhone after the first Mac, choose "server"

*I'm really serious about this, because I haven't tested this procedure.