I've looked at this article- HipHop PHP (was Hyper PHP by Facebook)
However, recently come across this Facebook Speeds Development With “HipHop Virtual Machine”, A 60% Faster PHP Executor
Does anyone have details on this? Is it worth exploring for a PHP developer?
You can find the details in the article linked in the article you linked:
The type-specialized machine code runs faster. Unless you are the size of FB, you do not need this. Use APC or memcached and more traditional approaches to scaling out.
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.