I was afraid that the usefulness of HipHop would be as limited as is regarding that it's not an easy feat to create a PHP-to-C++ compiler that handles C library dependencies (which PHP has a lot!) well.

BTW It was the second time in a week that there was a product that created incredible buzz in HN community without anyone able to trying out the product (the other was iPad of course) and I was amazed at the amount of well-informed opinion based on such little information.

PS. This blog is a good reading if you are interested in Facebook architecture, scaling and design issues in general.