The autocomplete plugins disables the use of the up and down arrow keys. In a single line input area this is not an issue, but on a textarea it is very noticeable (especially if you are a poor-spller like I am). I think the arrow keys should only be overridden when the autocomplete "drop-down" is visible.
As an example fill the "Multiple Birds (remote)" example at http://jquery.bassistance.de/autocomplete/demo/ with "Birds can be fuund all over the wurld, some such birds are:" now try and use the arrow keys to go back and fix the errors.
