Stuart, having been a general contractor for over 20 years I can tell you that local codes in my neck of the woods require that any opening in a load bearing location of the foundation wall 24" or greater requires either (1) that the rim joist be doubled across the opening extending a minimun of 3" past the edges (just like a standard header in a framed wall) of the opening and any adjacent floor joists falling within the span in question be secured with appropriate joist hangers, or (2) the opening must be spanned by an engineered LVL with the same rules applying concerning the overlap and joist hangers.
Could be different in your area, but I promise the builder would be a very unhappy camper if I caught him trying it on my new house.