ajax :update option with Rails 3

With Rails 2.3.9, I could use the :update option with an ajax helper
if I just wanted to update the contents of an element. The controller
action looked liked this:

format.js { render :parital => '_updated_object', :layout => false }

I'm trying to figure out how to accomplish the same thing with Rails 3
ujs? I know I can accomplish this by having the controller render an
function to update the html, but I would rather define that in my
application.js. Does that make sense?

Thanks,
Tom

I haven't heard a reply and am wondering if I wasn't clear or just
totally in left field with the question?