As far as hit rating goes: Yes, it's very important! I find that hit rating really makes the biggest difference in effectiveness for almost every non-healer in game. You need hit rating to hit stuff, it's that simple. If you don't have it, you're not hiting the mob, and nothing else you do will amount to much.

For equal level mobs, I believe the base miss percentage on a two-handed weapon is around 5%, so I like to try to keep to 5% when leveling, grinding, or 5-man grouping. For raiding, that bumps up to 9% for a 2-hander, whereas dual wielding is 28%.

The other problem is that spell hit's miss rating is 17% for raiding/boss mobs. Luckily, if you have Virulence specced and a Shadow Priest casting Misery on the mob, that drops down to 11%.

At the least, you'll want to hit that 9% 2-handed cap for raiding, which should be about 296 hit rating, I think.

On the Dual Wield issue, I will honestly be 2-handed tank until the day I quit playing a Death Knight. I just don't like all the extra hit rating and expertise a dual-wielding tank would need to keep from getting parry gibbed and be able to actually hit a decent amount with his off-hand. I think that 2-handed tanking will win out as the superior tanking style in the end, simply because any extra defense or parry or dodge you get on 2 tanking two-handers will be negated by loss of threat from lower damage weapon strikes and having to replace defense stats on your armor with hit rating and expertise.