Demonology Warlocks are about as good tanks as Shamans. With Dark Apotheosis and Nether Plating, they do get some okay armor mitigation (50% in my gear), plus they can taunt and heal and stuff. They're still not crit immune, though, which can lead to a fiery death real fast.
That said, I had fun tanking Stonecore when the tank got knocked into the adds on the Ozruk dude. Jumped in with my Immolation Aura, Hand of Gul'dan and Void Ray (still can't believe they'd use that name for it) and tanked the crap out of that stuff!
Wouldn't recommend it in raiding, though.