WordPress 3.4.2 maintenance and security release now available. Please update your sites! http://t.co/LTNlGHEY
— WordPress (@WordPress) September 6, 2012
I`ve updated all my networks with no problem, so if you did everything by the book(didn`t edit the wordpress files). Feel free to backup(just in case) and update. The process won`t take more than 1 minute, but it`ll fix some major security issues for wpmu, so I strongly encourage you to do it asap.