Update: the upgrade was rolled back until further notice, due to XML crashes. We’ve upgraded HHVM from version 3.3.0 to 3.4.2. Functionally speaking, this is a transparent upgrade for Magento. However, this solves an important memory leak issue, which in some cases could eat all of a server’s memory, causing it to crawl to a halt. No more! HHVM memory consumption is limited now.
Also see the Facebook blog about the new HHVM release.