Term reference field: Checkboxes with add new textfield
http://drupal.org/node/1208512#comment-5605324

It has been adapted from Select or other to support taxonomy term references.

It is a non-contrib module and you may need to tweak it to your taste, especially as it adds Other: to the term you have added. I just tried it out and that part needs to be fixed, but it shouldn't be difficult.