Help with my plugin and variables?

I am writing a plugin, and am having some problems. I want users to set their username in the script. The script so far is:

<?php
/*
Plugin Name: TweetPress
Plugin URI: http://www.podtube.co.cc/ex/tweetpress
Description: Twitter on the side.
Version: 1.2.9.2
Author: Zack Fulpington
Author URI: http://www.podtube.co.cc/
*/
$twitter_username = "ziggyzack99";
add_action('plugins_loaded', 'tpinit');
function tpinit() {
	if (!function_exists('register_sidebar_widget')) {
		return;
	}
	register_sidebar_widget('TweetPress', 'tweetpress_function');
	register_widget_control('TweetPress', 'tweetpress_menu');
}
$broom = isset($_POST['username']);
function tweetpress_menu () {
	?>
	<p style="text-align:left"><label for="username">Twitter Username: <input style="width: 200px;" id="username" name="username" type="text" value="<?php echo $username; ?>" /></label></p>
	<?php
}

function tweetpress_function($args) {
$twitter_username = "ziggyzack99";
	echo $args['before_widget'];
	echo $args['before_title'] . 'Twitter' . $args['after_title'];
?>
<div id="twitter_div"><ul id="twitter_update_list"></ul></div>
<script type="text/javascript" src="http://twitter.com/javascripts/blogger.js"></script>
<script type="text/javascript" src="http://twitter.com/statuses/user_timeline/<?php echo $twitter_username?>.json?callback=twitterCallback2&amp;count=1"></script>

<?
	echo $args['after_widget'];
}
?>

Now, at the top, you define $twitter_username. Then, at the bottom, the sidebar widget should output it so it shows your twitter feed with javascript. Everything works fine and dandy, except the echo does not work! Why is this?