Adding a Prologue-like edit box

I’m afraid I’m a complete newbie to hacking wordpress (although not to coding more generally), so I’m sorry if this question has an obvious answer.

I’d like to add a text field to the top of my site to allow logged-in users to write very quick blog posts (ideally in a default category).

The code in Prologue that displays the form is (I’ve removed a couple of minor things just to make it clearer):

<?php
$user			= get_userdata( $current_user->ID );
$first_name		= attribute_escape( $user->first_name );
?>

<div id="postbox">
	<form id="new_post" name="new_post" method="post" action="<?php bloginfo( 'url' ); ?>/">
		<input type="hidden" name="action" value="post" />
		<?php wp_nonce_field( 'new-post' ); ?>

		<label for="posttext">Whatcha up to?</label>
		<textarea name="posttext" id="posttext" rows="3" cols="60"></textarea>

		<label for="tags">Tag it</label>
		<input type="text" name="tags" id="tags" autocomplete="off" />

		<input id="submit" type="submit" value="Post it" />
	</form>
</div> <!-- // postbox -->

But placing this code on the page doesn’t seem to work. The form is displayed but submitting it does nothing. What do I need to do to get it to work properly?

Best wishes,

Nicholas