Look in includes/languages/english/YOUR_TEMPLATE/index.php (where YOUR_TEMPLATE is the name of your custom template) and change it there.

Code:
// Showcase vs Store
if (STORE_STATUS == '0') {
  define('TEXT_GREETING_GUEST', 'Welcome <span class="greetUser">Guest!</span> Would you like to <a href="%s">log yourself in</a>?');
} else {
  define('TEXT_GREETING_GUEST', 'Welcome, please enjoy our online showcase.');
}
If you don't have it in includes/languages/english/YOUR_TEMPLATE/index.php, copy index.php from includes/languages/english/.