HipHop Virtual Machine (dead link) says that HHVM can run WordPress.
A more technical description of the HHVM can be found in Speeding up PHP-based development with HipHop VM. One of the main targets was to be as fast as the previous PHP to C++ translater. This goal has been achieved, see graph:
Unfortunately this also shows that Facebook has bundled all their energy on improving the virtual machine, but failed to improve the PHP to C++ translator.
Addendum 09-Dec-2013: See HipHop for PHP in Wikipedia. Performance has improved again. There still seem to be some issues with WordPress unit-tests.
Addendum 20-Dec-2013: HHVM: We are the 98.5% (and the 16%).