Twitter-style autocomplete in textarea

I am looking for a Javascript autocomplete implementation which includes the following:

• Can be used in a HTML textarea
• Allows for typing regular text without invoking autocomplete
• Detects the @ character and starts autocomplete when it is typed
• Loads list of options through AJAX

I believe that this is similar to what Twitter is doing when tagging in a tweet, but I can't find a nice, reusable implementation.
A solution with jQuery would be perfect.

Thanks.