Or you could just use
'#type' => 'text_format' and set the
'#format' => 'full_html' for the textarea.
Just keep in mind that the data stored in the field will be stored as an array with 'value' and 'format'. So when setting up your form you'll want to use something like this:
$prev_input = variable_get('my_ckeditor_field',array('value'=>'<b>booyah!</b><br/>Works as designed.', 'format' => 'full_html'));
$form['my_ckeditor_field'] = array(
'#type' => 'text_format',
'#title' => 'CKEditor Field',
'#format' => $prev_input['format'],
'#default_value' => $prev_input['value'],
Hope this helps anyone else looking to quickly and easily enable the wysiwyg of their choice on their FAPI textareas.
If you are attempting to change a plain text textarea generated by a module that is not your own, such as the Webform or HTML Mail modules, then use nicholas' method since it's your only option.