Do you imagine a future where Hack will merge back into PHP (like PHP 7), in the same style that Beryl & Compiz then rejoined? Or does the team intend for the two to always be adjacent-yet-separate?
I think the most likely outcome is that PHP will adopt some of HHVM's additional features, but remain a separate project. IMO that's a great outcome, since we'll both likely drive the other to be better.