How can I get a user_reference type field working that allows me to enter names that don't exist on the site as well as ones that do? So essentially I want a text field to operate in a similar fashion to the facebook tagging text field. By default I would be looking up other users on the site. So user reference field is obvious. However if the user isn't found I'd still like to keep the text they entered as they are just entering someone's name.  Obviously I could just go and code this I'm just wondering whether anyone has done or seen this done before?
Autocomplete username looks like it does the auto complete bit for user registration. It's fairly simple. And could probably be adapted for your UI needs. image annotate looks like it does something similar, but without the ajax.

If you end up making it and publishing it, you can always post it here as an answer.