There's no fundamental incompatibility. Hack files don't use <?php tags, they use <?hh tags. I'm sure Facebook will make HHVM handle PHP type declarations alongside Hack type declarations, though exactly how they'll interact remains to be seen.

This is going to depend on how Facebook decides to maintain their PHP spec. They might decide to fork it for all we know.

But honestly, if you're using HHVM already, you should have as much tooling/documentation as you need to switch to Hack itself within the next year or so, so all this PHP discussion is moot.