The default theme has been discussed a couple of times before here on the forums. Open up /wp-admin/upgrade-schema.php for editing, find the following lines:
add_option('template', 'default');
add_option('stylesheet', 'default');
And change the 'default' to the name of the subdirectory of the theme you want as your new default.
I added it to the wiki by the way.
To add in your pages, categories, and posts, ytou'll need to edit your wp-includes/wpmu-functions.php file and add in the MySQL statements. Do a search for 'Default category' and you'll see how it's done.
Hope this helps,
-drmike