@evilbob you just don't wont to see the easiest answer, don't you
On p.61 in DMG in Obscured Terrain you read:
Obscured terrain provides concealment and blocks line of sight if a target is far enough away from you. However, it has no effect on movement.
So. Fog (vapors), heavy fog and darkness are obscuring terrain that block line of sight. Fog (light obscured), heavy fog (heavy obscured), darkness (totaly obscured). True?
Stinking Cloud creates a vapors. In description you see thats a thick cloud of bilious yellow vapors. Well. You have two options here. It's definetly not the totaly obscured terrain (it's not a darkness). So it's a light or heavy obscured terrain.
Later on p.61 in DMG you have:
Lightly Obscured: Squares of dim light, fog, smoke, heavy snow, or rain are lightly obscured. Taregt have concealment.
Heavily Obscured: Squares of heavy fog or heavy smoke are heavily obscured. Adjacent creatures have concealment. Far away are not seen.
And in other wizard powers you have one power that says about total concealment.
It's 23 Encouter Spell "Acid Storm":
The cloud blocks line of sight, providing total concealment to creatures inside it. Any creature that enters the cloud or starts its turn there takes 10 acid damage. The cloud lasts until the end of your next turn, or you can dismiss it as a minor action.
Don't forget that the Cloudkill on 19 Daily Power don't says about total concealment just like Stinking Cloud dosen't. It just block line of sight.
So whe have Stinking Cloud, Cloudkill, Fog Cloud (6 lvl Wizard Daily thats says fog gives concealment), Hunger of Hadar (darkness) and Acid Storm (cloud with total concealment). Well in almost all cases we have clear answer. Only in those 2 clouds we don't have. Ahh theres a Wall of Fire, Wall of Ice. Both bloking line of sight. Is fire so obscured it gives total concealment? I don't think so. So why clouds? I prefer that all they gives concealmen (-2 to hit). But it will be still fair to rule that are heavy obscured squeares giving total concealement (-2 adjacent, -5 non-adjacent). I hop that WoTC will give us answer soon.