I used the instructions at
http://forums.macosxhints.com/showthread.php?t=80054
Note that the directory:
http://yourhost/ical/ is where webdav requests go to. Your ical and omnifocus info can co-exist in there fine.
In the "sync" preference, I just put
http://yourhost/ical/ and OF created a document called "OmniFocus.ofocus" that it uses for syncing.
I guess to sync different ofocus documents, I'd need to set up more directories, but the above seems to work for me w/o too much effort.
I do recall having to wipe my databases at some point. Not sure if that was necessary, but I thought it was at the time!