By process of plug-in elimination I have discovered that v4.5.1 is crashing Cubase on loading a project with instances of H4 on 2 different Instrument Tracks (Real Fingered Bass).
Rolling back to 4.0.1 has solved the problem. I have an Apple Problem Report if Steinberg is interested in looking at this issue for future updates.
Running Mac 0SX 10.6.5.
(I tried to post a support request and got a 404 error upon Submitting.)