You have a CI injected 700 series car? What market are you in? As far as corrossion goes that one is fairly advanced. It looks like you actually have a rust colored substance in the bottom of where the t/stat goes, that is tough to acheive on an aluminum head. I often times will see a darkening of the metal when I change them out but just as often it will be clean, sliver colored aluminum. The thermostat housings corrodind away is pretty common in may market (So. Cal) so I would have to think that it is even more prevalent in the harsher climates. When they rot away though they just leave that white fuzzy looking crap, much akin to the stuff that you see growing on battery terminals. It is just aluminum oxide instead of iron oxide, same thing only different. Don't know what to tell you about your level of corrossion other than it can't be a good thing.

Mark