$(function() {
var theSearchBox = $('#searchBoxOuter td.ms-sbcell input');
var defaultSearchText = "Search this site";
var preferredSearchText = "Search my custom site name";
theSearchBox.val(preferredSearchText); //replace the text initially
theSearchBox.blur(function() { //replace the text when the search box loses focus and no value entered
if ($(this).val().indexOf(defaultSearchText) > -1)
$(this).val(preferredSearchText);
});
});