Update Rollup 12 has been partially released, but not the server parts at this time. While many of us have been waiting for the functionality provided in UR12, I recommend that you hold off or at the very least, perform thorough testing before applying this rollup into production enviroments, even after the release of the missing bits.
In addition to the issues with CRM 4.0 jscript on new browsers, this is a major update to CRM and I would tread carefully and slowly to protect your users form potential issues. Read all of the notices carefully before proceeding.
We are currently recommending that our clients install UR11 and hold for now unless there is a major need to move forward.