Hi phenomeno,
The easy way to set the search… text in the search field module is using ‘placeholder’ property which support on most modern browser.
Open file templatesja_wallhtmlmod_searchdefault.php and add placeholder=”Search…” to the search field.
Like this
$output = '<label for="mod-search-searchword">'.$label.'</label><input name="searchword" id="mod-search-searchword" maxlength="'.$maxlength.'" class="inputbox'.$moduleclass_sfx.'" type="text" size="'.$width.'" placeholder="Search..." />';
You can use other javascript plugin to make it more compatible with old browser, but I think it is not necessary.
https://github.com/mathiasbynens/jquery-placeholder
Regards