It’s been discussed since June, 2009 when Matt Mulliweg mentioned WPMU would be discontinued, or did he say merged? Well this has been the subject of much debate. Is WPMU going to be subsumed by WordPress? Will they become the same thing? Will both WordPress single-user and and WordPress multi-users (MU) both end up upgrading to the same thing?
http://wpmu.phreeblog.com/2009/10/14/the-future-of-wordpress-mu-%E2%80%93-have-your-say/
http://bavatuesdays.com/wpmu-state-of-the-union-here-and-abroad/