Changing the default site of a WordPress multisite/network installation
from http://wordpress.org/support/topic/default-multisite-redirect-to-one-particular-blog?replies=8
1) In Network Admin -> Sites and edit the site you want to be the new ‘main’ domain so it’s url is http://example.com (remove the subdomain or subfolder path totally).
2) Change the define(‘BLOG_ID_CURRENT_SITE’, 1); to whatever you want. Save it, upload it. DO NOT CHANGE SITE ID, YOU WILL BREAK THINGS.
3) Go BACK to Network Admin -> Sites and edit the site of the ‘OLD’ main site. Edit it so it has a sub-folder pathing. /edition2011/ or whatever you want. Save it.