Miss chance
Dual wielding adds a 19% miss penalty to both main and offhand weapons. The base miss rate for all characters is 5%, so each weapon by itself has a 24% chance to miss (19% + 5%). The talents Dual Wield Specialization for shaman, Precision for rogues and warriors, and Nerves of Cold Steel for death knights increase the chance to hit with both weapons while dual wielding.
For each point the attacker’s weapon skill exceeds the target’s defense skill, the miss chance decreases by 0.04%. For each point a target’s defense skill exceeds his opponent’s weapon skill, the miss chance increases by 0.04%.
Important note: Prior to version 1.8, dual wield's miss chance had a hard cap of 19%, meaning that all dual-wield auto-attacks had a minimum 19% miss chance regardless of how much +hit% gear was equipped. In version 1.8, this hard cap was removed. The exact wording of the 1.8 patch note was:
"Items which provide +hit chance will now be allowed to counteract the increased miss chance penalty of dual-wielding."
Take a level 70 rogue for example vs. a level 73 raid boss: the rogue has 5% + (0.04% * 15) = 5.6% base miss chance (assuming he has 350 weapon skill and the boss has no Defense skill bonuses), plus 19% dual wield miss chance, leaving us with a total miss chance of 24.6%.
Note that the +19% miss chance for dual-wielding only applies to ordinary auto-attack melee attacks. It does not apply to any special attack powers that cost rage, energy, or mana -- neither instant attacks such as Sinister Strike or Stormstrike, nor on next swing attacks such as Heroic Strike, incur the dual wield miss penalty. So, with 9% +hit from gear/talents a dual-wielder will never miss with an instant attack (although the attack can still be dodged, parried, or blocked -- see the Attack table article). However, a dual-wielder would need 25% +hit if he wants to remove the miss chance for auto-attack/white damage totally.