User can add any number of fields form frontend product view page.
There is a button to add more field. which user can add text and submit. Client wants to sale their mugs and each mug they want to save different message if they want.
To achieve that goal you have to use custom code, since Magento doesn't provide the functionality to do it via admin panel.
You will need to create a little module that adds the attribute and renders the input. There is an article that explains it: http://inchoo.net/magento/adding-magento-attribute-with-custom-input-renderer/