I come up with this after 20s of googling:
text_field_with_auto_complete foo, bar, :tag_options => { :value => "some default text" }
Doc for text_field_with_auto_complete is here http://api.rubyonrails.org/classes/ActionView/Helpers/JavaScriptMacrosHelper.html#M000579 , the tag_options hash can hold the same stuff as the options hash in text_field_tag ActionView::Helpers::FormTagHelper .
Felix