You can upgrade from 2.6.5 quite easily, just:
1) backup eveything
2) delete your WP-admin and WP-includes
3) delete all files in the root directory (I prefare leaving the wp-config, and not delete this file)
4) add these two lines:
define( ‘NONCE_KEY’, ‘HStKtEP!B&KF8RVr!FgrVoMM’ );
define( ‘AUTH_SALT’, ‘sGoBeog@D8KB91I7oeCu&e(&’ );
/* That’s all, stop editing! Happy blogging. */
5) and you’re done.
tell us what you think of the new version.
some more information about WordPress Mu 2.7:
the timeline – http://trac.mu.wordpress.org/timeline
from the blog of Donncha O Caoimh – http://ocaoimh.ie/2009/01/28/wordpress-mu-27/
from the WordPress Mu official forum – http://mu.wordpress.org/forums/topic.php?id=10163