Need help with observe_field

Hey all,
I’m trying to update a div everytime the value of my textbox is changed:
here is _form.rhtml:

Address
<%= text_field 'bookmark', 'address' , :size => 120 %>

<%= render :partial => 'bookmarktitle' %>
<%= observe_field 'bookmark_address',:frequency => 0.5, :update => 'ajaxtitle', :url => {:action => 'bookmarkaddress' }, :with => "'url=' + bookmark_address.value" %>

and this is the updated part _bookmarktitle.rhtml:

Title
<%= text_field 'bookmark', 'title' , :value=>@title, :size => 120 %>

the function bookmarktitle:
def bookmarkaddress
@title = “test value”
end

However when I modify champ bookmark_address, nothing happens.
Any idea what’s wrong in my code?

Thanx in advance

Pat