I finally figured out how to extend Wordpress 3.1 with an Itunes RSS (http://itunes.apple.com/gb/rss/topsongs/limit=10/xml). I'm not sure if it's the perfect solution but it works and that's the main point.

1: Create an .inc file in your theme folder "simplepie-itunes.inc.php"

We are going to extend the SimplePie class. This is pritty easy and works the same for any other RSS feed you want to extend. Define the NAMESPACE from the RSS feed.

<feed xmlns:im="http://itunes.apple.com/rss" xmlns="http://www.w3.org/2005/Atom" xml:lang="en">  </feed>

Fetch the "xmlns:im=" url: http://itunes.apple.com/rss"

<?php
define("SIMPLE_NAMESPACE_ITUNES", "http://itunes.apple.com/rss");  

Extend the SimplePie_Item with a new Class "SimplePie_Item_Itunes".

 
<?php
class SimplePie_Item_Itunes extends SimplePie_Item {  
	...
}

To get the data from the tags in the feed, will need to define the functions that we are going to use in the Wordpress theme. Find the tag in the feed and copy the part after "im:" in the tag. Define the functions. The name of the functions is up to you, you can name it like you want. The functions will output the data as a string if i got it right.

 
<?php
class SimplePie_Item_Itunes extends SimplePie_Item {  
	function get_artist_name() {  
	    	$data = $this->get_item_tags(SIMPLE_NAMESPACE_ITUNES, 'artist');  
    		return $data[0]['data'];  
	}
	
	function get_song_title() {  
    		$data = $this->get_item_tags(SIMPLE_NAMESPACE_ITUNES, 'name');  
    		return $data[0]['data'];  
	}  
	
	function get_tag_name_of_choice() {
		$data = $this->get_item_tags(SIMPLE_NAMESPACE_ITUNES, 'tag_name_of_choice');
		return $data[0]['data'];
	}
}

That's pretty easy until now. The rest is simple as well but it took me some time to figure it out. "Call me stupid, but you are searching as well."

2: Add the RSS to you template

We are going to add the feed to the theme with core code from Wordpress 3.1. But instead of loading the "feed.php" file which the Wordpress Docs tell you to do, we include the "class-feed.php" file instead. Don't know why it doesn't work with the "feed.php" file but somehow the you will get an error telling you it can't extend the "SimplePie_Item" Class.

	<?php
	// include the SimplePie Class
	include_once(ABSPATH . WPINC . '/class-feed.php');
	// include uour custom Class extension for Itunes
	include_once(TEMPLATEPATH. '/simplepie-itunes.inc.php');
								
	// Get a SimplePie feed object from the specified feed source.
	$feed = fetch_feed('http://itunes.apple.com/gb/rss/topsongs/limit=10/xml');
	
	// Add the extende SimplePie class for Itunes
	$feed->set_item_class('SimplePie_Item_Itunes');
	
	// Checks that the object is created correctly
	if (!is_wp_error( $feed ) ) : 
		// Define the max items (In this feed there are only 10) 
	    $maxitems = $feed->get_item_quantity(10); 
	    
	    // Array of the feed.
	    $items = $feed->get_items(0, $maxitems); 
	endif;
	?>

We have now included the feed and extended the SimplePie Class.

3: Print the feed items in the theme, in a nice table

To print the artist name and song name we are going to use the extended class functions: "get_song_title()" & "get_artist_name()".

	<h1><?php __('Itunes Chart'); ?></h1>
	<table summary="Itunes top 10 UK">
		<thead>
			<tr>
				<th class="first"><?php __('Posistion'); ?></th>
				<th><?php __('Artist'); ?> & <?php __('Song title');?></th>
			</tr>
		</thead>
	   	<tbody>
		    <?php 
		    // if the feed is empty
		    if ($maxitems == 0) : 
		    ?>
		    	<tr><td>No items.</td></tr>
		    <?php endif ?>
		    <?php
		    // Loop through each feed item and display each item as a hyperlink.
		    $i=1;
		    foreach ( $items as $item ) : ?>
		    <tr>
		    	<td><?=$i;?></td>
		        <td>
		        	<a href="<?php echo $item->get_permalink(); ?>" title="<?php echo $item->get_date('j F Y | g:i a'); ?>">
		        		<!-- get the song title -->
		        		<strong><?php echo $item->get_song_title(); ?></strong></br />
		        		<!-- get the artist name -->
		        		<?php echo $item->get_artist_name(); ?>
		        	</a>
		        </td>
		    </tr>
		    <?php $i++; ?>
		    <?php endforeach; ?>
		</tbody>			
		<tfoot>
		</tfoot>
	</table>

Splitting the code up. Most is straight forward. We create a table with a table heading. The the tbody part is the where all the action is at. First a message when the feed didn't load or is empty. Then the actual parsing of the feed items. Here is where you will use you custom class functions.

<?php foreach ( $items as $item ) : ?>

Run through the array of items

<?php echo $item->get_permalink(); ?>

Link to iTunes store.

<?php echo $item->get_song_title(); ?>

Your custom class function for getting the song title

<?php echo $item->get_artist_name(); ?>

Your custom class function for getting the artist name

That's it. Easy does it, you can extend the class off course to get the album image or any other data from the feed. Good luck!