Hrms. I'm not 100% convinced, but the points you guys bring up is why my post originally hedged my bets and said the area was somewhat murky.
Frex, the power you reference says:
Hit: 1[W] + Strength, Dexterity, or Constitution damage + 2d6 necrotic damage. You regain hit points equal to twice the necrotic damage you dealt.
It would seem like all of the damage is necrotic, although the second line explicitly calling out the necrotic damage seems weirdly worded in that case ...
More generally, I *think* the errata that changed Vulnerability/Resistance to how they currently work with re: to greater than one damage type also changed how powers with more than one damage type worked. I'll try to look it up when I get home ...
I fully admit I could be wrong though! So maybe you shouldn't listen to the bug headed guy. :P