The other answer about HPHPC is accurate... the performance boost is quite nice, but only if you actually need it enough that it's worth the extra complexity. 
I'd suggest waiting a few weeks or even months before looking into HPHPVM.  It's looking promising, but it's still in the very early stages as far as optimization and bug-fixing go.  In the long run it'll probably end up being a great alternative to Zend, but right now the relatively small boost in performance (compared to, say, full translated/compiled HipHop) is likely not worth the extra complexity.  That said, do check back in a few months.