That argument doesn't really make sense, since you can run JavaScript / Objective-J on both the client and the server, but you can only run PHP on the server (though from what I understand you translate PHP to JavaScript?)
Objective-J runs on a number of JavaScript engines that can run on the server, including Rhino (check out the "objj" executable in the "Tools" package: http://cappuccino.org/download/). While we don't have a good web server solution yet, it's certainly a possibility.
But even once that is a reality, Cappuccino will be totally server agnostic, so it will work with whatever server side technology you are comfortable with (or stuck with)