svn co http://svn.automattic.com/wordpress-mu/branches/2.7 ./wpmu
cd wpmu/wp-content/plugins
svn co https://svn.buddypress.org/trunk ./buddypress
svn co [my-plugin-repo-url] [my-plugin-dir]
Then when you want to update WPMU just svn up in /wpmu/ and when you want to update BuddyPress just svn up in /plugins/buddypress/
