Actually at high gear levels DKs are fantastic tanks because they self heal and have a shit-ton of dodge/parry with very high armor. If I wipe my rets accidentally, say on Anub with pound, my DK can almost solo the rest of the fight (depends on which burrow phase my rets died.)

Just a word of warning: All tanks are changing in Cata. Don't expect your paladin to easymode heroics because he can block. It's no longer a static value (i.e. block 2k off a hit) but a percent. So you will be more like DKs and Druids. Also DKs are getting an absorb shield that procs so they are more like druids. I think this will make them fantastic tanks, but then we won't know until Cata ships.