The standard 8% for special attacks applies to DK tanks. Pick up Nerves of Steel (Frost Tier 2) reduces Dualwielding to 5%. The Draeni racial stacks on top of gear, though your character sheet won't show it.
Note that spell abilities like Icy Touch have a higher need on hit, but there was discussion on where that cap actually fell. I personally don't see misses while running at 8%, so it may have been stealth changed since WotLK's release.