Not tested, could use some help to stabilize
This fork is meant to work with acts-as-taggable-on and creates new tags if they aren't found.
IT USES TAG NAMES not IDs and should work with <%= f.text_field :tag_list %>
I have found my changes to be slightly buggy at times...
Example implementation with acts as taggable: https://gist.github.com/988968
jQuery Tokeninput: A Tokenizing Autocomplete Text Entry
Tokeninput is a jQuery plugin which allows your users to select multiple items from a predefined list, using autocompletion as they type to find each item. You may have seen a similar type of text entry when filling in the recipients field sending messages on facebook.
Documentation, Features and Demos
Full details and documentation can be found on the project page here: