WTF: How do I get Drupal to put placeholder text inside my textfield?
What's that feature: the #attributes key.
$form['my_textfield'] = array( '#type' => 'textfield', '#attributes' => array( 'placeholder' => t('My placeholder text'), ), );
Take note that this is telling Drupal to use the placeholder="My placeholder text" attribute for the textfield. The placeholder attribute is an HTML5 attribute. Keep this in mind when considering what browsers will be accessing your site and whether it's important for all visitors to see the placeholder text.
You can tell Drupal to use any (valid) HTML attributes with this technique.
This really-easy-to-use feature was one that I had forgotten about. So, I figured others may have forgotten about it as well. I hope you enjoy it!