The Doctrine project has posted an update about the work being done in collaboration with and to help its performance with HHVM (the HipHop VM from Facebook) and talking about their future plans.
Facebook has been pushing HHVM alot lately, helping open source projects to get their test-suite running 100%. For Doctrine HHVM is particularly interesting, because of the performance gains that the complex PHP algorithms inside ORM would probably get. From my current feeling Doctrine will be the PHP open-source project getting the most gain from running on HHVM. However with the tests not yet passing on the ORM, we can only imagine how big that performance improvement will be.
One of their goals is to be able to run DBAL/ORM on HHVM with 100% passing tests. So far they've been working on Common project functionality and have three as fully supported under HHVM - Collections, Inflector and Lexer. Work is still being done on other parts of the codebase, with the ORM and DBAL being the lion's share of the job.