If you remain inside your tribal border, then you are opting out of PVP. Given you can build defensively and keep out or at least create choke points, what's the problem with PVP again ?
I think this whole Safe Zone Enabled/Disabled is going to be a headache and just complicates things as mgilbrtsn has already suggested. But, I'm thinking one way around it is how the corporation warfare is handled in EVE Online. You have to declare war with the corporation before you can attack them (in high security regions with police presence), and there's maintenance costs to keep the war going. Rather then the annoying Safety Zone Enabled/Disabled nonsense, let tribes declare war on one another before they can attack eachother? But if you have a tribe declaring war on you and you don't want to, allow tribes to build safe houses so members have some form of safety?
Keith I read from Xsyon that the current engine/game does not support land (server) addition like in EvE. Did you read somewhere else that it will in the future?
During the Prelude, Xsyon will continuously evolve and expand. Denizens will uncover remnants of lost technology, discover new crafts and skills and encounter new creatures. A deadly fog which surrounds the basin will slowly recede to expose new lands, more affected and ravaged by the ongoing apocalypse.
120 sq km starting playing field, gradually expanding to a 1200 sq km area and beyond
http://www.xsyon.com/features
I don't know if it's 120 sq km now or what...
*Edit*
Is the only feature people read "FFA OPEN PVP" and hit pre-order ?