I don't think so :) At least in IE, it's impossible to catch up and down...
What I did do is that <space> will autocomplete as much as possible and ! will set the focus to the first link, so you can tab up and down...
Wow :) It would be nice to hide the layer in case I change my mind and click into the document area for example. You can only make the layer disappear now with cleaning the input box...
Goba
