OK, the results are in and the problem was the PMTU discovery.  The
state of netaddr doesn't seem to matter on the O2 I'm testing however. 
Thanks to all that piped-up.

May I suggest the following addition to the installation notes and/or
FAQ, just after the section on net.inet.ip.anonportmax:

        If your boot host is running Linux the equivalent command is:
        
                # sysctl -w net.ipv4.ip_local_port_range="16384 32767"
        
        Additionally Linux usually has path-MTU discovery enabled which
        can cause problems on some machines, so you may need to disable
        this:
        
                # sysctl -w net.ipv4.ip_no_pmtu_disc=1
        
        Some people have also reported that using version 3 of the ISC
        DHCP server gives better results.

Cheers,
Steve