According to this new post on Nerds Central, there's been speculation that Facebook, needing even more of a performance boost than hiphop gave them, is moving into using PHP in a JVM.

The presence of Facebook engineers at the JVM Language Summit in San-Francisco along with their interest in implementing PHP using invoke-dynamic on the JVM is a the shock. The main seismic event will be nothing less than the complete removal of interpretors from main stream general purpose programming.

He talks some about the JVM environment (and the Magik project) as well as wondering about what sort of performance boost it might actually give.

So Are Facebook Doing The Right Thing? Yes! The rise and rise of Javascript over the last 4 years as shown us just how powerful JIT compilation of once interpreted languages can be. The fall and fail of complete re-writes has shown us just how unrealistic it is to completely move a working system from one language to another. Facebook has a stupid amount of PHP and so it is by far the most sensible thing to port that to mlvm. Actually it should be pretty easy.