All right,, let's talk about Clark first. I agree with PG that the decision was a relief. I am not sure that the Constitution contains any sanity requirement at all, let alone the one that was at issue in this case. Clark's due process argument really came down to the 70's-era debate about state-granted rights. Clark's complaint was essentially that having created a mens-rea-type element of a crime, Arizona had to give him a chance to prove that he didn't have it (and had to give him the chance to do it in this particular way). The first response one is tempted to make is that since Arizona doesn't have to make mens rea a requirement at all (murder could be a strict-liability crime), it is entitled to define and establish proof of mens rea however it darn well pleases.
True, that response is too quick by half. At its limit, it would allow the state to avoid any constitutional procedural protection. That is why substantive due process is intuitively joined with procedural due process, for better or worse. But anyway, no need to go that far in this case. This seems to be a bang-up job by Justice Souter.