OK, here's a brain teaser...
I reckon WPMU should be installed so that the main dashboard feed pulls in the latest posts from the main site (mywpmuinstall.com) and the planet pulls in latest posts from around the site - using the wpmu feed install.
To get this I figure you need to edit index.php but my php skills aren't such that I can figure out how to call the 'home' url of the wpmu install into fields such as the below:
<?php
$rss = @fetch_rss('http://wordpress.org/development/feed/');
if ( isset($rss->items) && 0 != count($rss->items) ) {
?>
<div id="devnews">
<h3><?php _e('Latest site news'); ?></h3>
<?php
$rss->items = array_slice($rss->items, 0, 3);
foreach ($rss->items as $item ) {
?>
<h4><a href='<?php echo wp_filter_kses($item['link']); ?>'><?php echo wp_specialchars($item['title']); ?></a> — <?php printf(__('%s ago'), human_time_diff(strtotime($item['pubdate'], time() ) ) ); ?></h4>
<?php echo $item['description']; ?>
<?php
}
}
?>
Any help would make my weekend a very happy one :)
Cheers, James