This is the example code used in the "Fields in Core: How to create a custom field" presented at Drupal Downunder 2011 (http://drupaldownunder.org/session/fields-core-how-create-custom-field)

Presentation slide can be found at: http://www.slideshare.net/zugec/fields-in-core-how-to-create-a-custom-field

Example code only, please review for any security issues.