The state of the art in JITs is LuaJIT2, despite almost no fanfare (and lots of fanfare by other JIT projects). I haven't had a look at hhvm yet, but if anyone wants a cool project, make hiphop emit Lua instead of C++, and run it under LuaJIT2.

LuaJIT2 really is magic, and Mike Pall is a humble but talented magician.