Basically they've used their exposed pipeline design and register metadata to fix the things that didn't work well about ALAT. Or at least that's the theory.

EDIT: I think the main problem was that on Itanium issuing a speculative load could potentially trigger a page fault making it potentially dangerous. On the Mill the page fault won't trigger until the load leads to a side effect outside the belt, sort of like it's been wrapped in a Haskell Maybe monad. So if you' have something like

  for (int i = 0; i < foo; i++) {
    a[i] = a[i]+1;
  }

you can speculatively load the a[i+n] while you're working on a[i] even if allocated memory stops at the border of the array allocation.