Rails - Apr 2006 - observe_field inside of a form?

Hi I''ve just started experimenting with AJAX in Rails and have come  
across this issue.

I''m trying to set up some ajax form validation on a login form. I set  
up an observe_field method for the username field but the action for  
the containing form is being called and rendered instead of the  
action I specify in the observe_field method.

All of the observe_field examples I''ve found are done using a single  
input tag, no containing form...is this the issue or is it something  
else I''m missing?

<%= form_tag %>
<fieldset id="login_form">
	<p>Please Log In</p>
	<p>
         <label for="user">Username:</label>
         <%= text_field("member", "username",
"class" => "text
required", "id" => "username") %>
		<div id="username_verify"></div>
	</p>
</fieldset>
<%= end_form_tag %>

<%= observe_field :username, :frequency => 0.5, :update  
=> :username_verify, :url => { :action => :verify_username } %>