How do I create an autocomplete text field that allows for multiple words/tags

I have described the problem in details in the above post. What I’d
like is for the autocomplete field to allow the user to enter multiple
‘tags’ or ‘categories’ separated by commas. e.g. “Ruby, Rails,
Currently I have auto-complete working for Comma separated tags…
However selecting tag#2 from the dropdown replaces all content in the
text field… only the last selected tag is shown.
Any ideas?