Donncha released an awesome WordPress MU plugin, Sitewide Tags Pages that creates a set of pages like the WordPress.com Hot Topics pages, but the magic is that it also feeds posts into a separate blog and creates a sitewide feed of all posts plus feeds for any tags and categories as well.

Unfortunately I could not get it to work with a prologue type theme because posting is performed in the front-end vs. the backend of WordPress, and the plugin relies on the /includes/taxonomy.php file located in the wp-admin directory. The error produced is:

Fatal error: Call to undefined function wp_insert_category() in /home/myname/public_html/wp-content/mu-plugins/sitewide-tags.php on line 120 or 148 in some cases.

After following a recommendation Donncha provided regarding foregoing the categories altogether. Considering that there are really no uses for categories in my opinion with Prologue or similar themes, I commented out lines 118-122 in the sitewide-tags.php file, and everything seems to work great!

Should look something like this:
118 //if( is_array( $cats ) && !empty( $cats ) ) {
119//foreach( $cats as $t => $d ) {
120//wp_insert_category( array('cat_name' => $d, 'category_parent' => '') );
121//}
122//}

I can now see post and tags on the Global site after posting from a Prologue themed blog, and still access the admin and perform normal functions on all site involved.

If you are running a WordPress MU 2.6.1 installation, I strongly recommend downloading Donncha’s latest version of the Sitewide Tags Pages Plugin.