A Harvard architecture can help in the same way NX does (as well as boost performance for fixed-purpose applications that rarely need to be re-programmed), but for it to still be Turing complete it's going to need some way to modify the executable code, including the potential for exploits, albeit Harvard architecture-specific ones.

The Harvard architecture could be used as the basis for a more rigorous trust model (iff the owner of the system controlled the root of trust.) Democracy is out of fashion though, so we would undoubtedly get something similar to what we have today with Redhat (for all practical purposes) "having to" pay for the right to boot Linux on a system "certified for Windows 8"...