I just had the exact same issue. There are no fields in the default search control to enable this. I just created some Javascript to insert/hide the default text.

Add the following script anywhere in your page. I just have it run at window.load--didn't bother with running exactly after the search textbox is parsed for DOM usage, but I didn't care since it's just unobtrusive help text.

Set the DTF.search.mainSearchTextboxClass to the CSS class that is set in the search control for the text box. Also, set the DTF.search.innerSearchHelpText to whatever text you want shown.

This code requires the ASP.NET AJAX library helper method of $addHandler. Feel free to replace that with your preferred add handler method.

var DTF = {
getElementsByClass: function(searchClass, node, tag) {
var classElements = new Array();
if ( node == null )
node = document;
if ( tag == null )
tag = '*';
var els = node.getElementsByTagName(tag);
var elsLen = els.length;
var pattern = new RegExp("(^|\\\\s)"+searchClass+"(\\\\s|$)");
for (i = 0, j = 0; i < elsLen; i++) {
if ( pattern.test(els.className) ) {
classElements[j] = els;
j++;
}
}
return classElements;
},

search: {
innerSearchHelpText: 'SEARCH',
mainSearchTextboxClass: 'MainSearchTextbox',

init: function (e){
// set initial value
var elem = DTF.search.getSearchTextElement();
if (typeof(elem) !== 'undefined' && typeof(elem.value) !== 'undefined')
{
//if (window.console) window.console.debug('Found the main search text box with class name "' + DTF.search.mainSearchTextboxClass + '".', elem);

if (elem.value == '')
elem.value = DTF.search.innerSearchHelpText;

// setup handlers
$addHandler(elem, 'click', DTF.search.focus);
$addHandler(elem, 'focus', DTF.search.focus);
$addHandler(elem, 'blur', DTF.search.blur);
} else if (window.console) {
window.console.error('Could not find the main search text box with class name "' + DTF.search.mainSearchTextboxClass + '".', elem);
}
},

focus: function (e){
var elem = DTF.search.getSearchTextElement();
if (typeof(elem) !== 'undefined' && typeof(elem.value) !== 'undefined' && elem.value == DTF.search.innerSearchHelpText)
elem.value = '';
},

blur: function (e){
var elem = DTF.search.getSearchTextElement();
if (typeof(elem) !== 'undefined' && typeof(elem.value) !== 'undefined' && elem.value == '')
elem.value = DTF.search.innerSearchHelpText;
},

getSearchTextElement: function() {
var elem = null;
var possibleElems = DTF.getElementsByClass(DTF.search.mainSearchTextboxClass);
if (possibleElems && possibleElems.length == 1)
{
elem = possibleElems[0];
}
return elem;
}
}
};
$addHandler(window, 'load', DTF.search.init);

if(typeof(Sys)!=='undefined')Sys.Application.notifyScriptLoaded();