When talking about OpenSim- and Hypergrid-security I believe several aspects have to be taken into account it is not enough to secure the server and leave the Hypergrid door wide open or vice versa.
On the local side basically all average grid owners can do, is to take basic measures like a firewall and ensuring their system is up to date. It would be advisable though to go a step beyond the usual and use programs like portsentry or fail2ban to thwart and block the casual hacker. If you really have a determined and knowledgable hacker there is not really much you can do to keep him or her out.