While trolling around the DNN forums, I found a post by someone looking to add the text "Search" to the DNN Search box. I've done this several times, and I use a javascript snippet. For those that are looking for something similar, see below.

 

$(document).ready(function() {
 $('#dnn_dnnSEARCH_txtSearch').val('Search Our Site'); //add the search text to the search input
    $("#dnn_dnnSEARCH_txtSearch").focus(function () { //clear the default text out of the search input
        if ($(this).val() == "Search Our Site") {
            $(this).val('');
        }
    });
});

This will add the text "Search Our Site" to the search box. When the user clicks on the box, the text is erased.

 

The best place to add this is in your skin files. Make sure you are using jQuery.