Hi! I would suggest a new Tag selection type that behaves exactly like Drupal's freetagging vocabulary and suggested terms: http://drupal.org/project/suggestedterms
I've noticed that there is a huge memory issue with Custom Field types such as: "Alphanumeric > Multiple select", radio buttons, or checkboxes when there are lots of items you could select. IMHO, an AJAX input field like in freetagging would be a great improvement...
Maybe we could implement Drupal's existing freetagging and vocabulary system into CiviCRM?