Set up a new NFS share in OMV .3 (latest and greatest).
Client set to 192.168.1.0/24 and extra options set to all_squash,insecure.
I simply then try a "Connect to Server" on my Mountain Lion installation, http://192.168.1.199, expect to get a finder window with the share, but instead get a dialog box saying "You do not have permission to access this server."
Are there some special config options I need to set on the NFS share in OMV to make this work?