Items: +Spell Hit Chance

From http://www.worldofwarcraft.com/info/basics/spells.html
In general, for 5 man and heroics you should aim for 4-5% spell hit, beyond that is not as valuable. Karazhan there will be lvl 73s so spell hit continues to be valuable beyond that.
Ex. Cheve (assume hes 70) has 2 pts in Shadow Focus which gives him +4% spell hit. That was easy. The value of spell hit now only pertains to 2% more against lvl 72s, so your item points may be better spent elsewhere.
Poor cheve at lvl 68 only had a 76% chance to hit the lvl 72 bosses in Shadow Lab last night.
+1% spell hit chance increases the base chance for a spell to land against a target by 1%

If your target is the same level as you, a spell has a base chance to hit of 96%.

If the target is +1 level compared to you: 95%

+2 levels: 94%
+3 levels: 83% if the target is a monster, 87% if the target is a player.
+4 levels: monster: 72% player: 80%
+5 levels: monster: 61% player: 73%
Etc...

If you wear a +1% spell hit chance item, the above percentages will increase by 1. +2% gives +2.
Etc...

For binary spells only, there is an additional modifier for the resistance of the victim to your particular spell school: fire, frost, shadow, nature, arcane. That modifier is multiplied by your hit chance to get your actual chance to land. This is done with binary spells only, because they never do partial damage.

Example:

Eyonix the Mage (level 60) fires a frost bolt at Yeti of Doom (level 63). Eyonix is also wearing a total of +6% spell hit gear. Yeti of Doom has frost resistance such that he takes 50% from level 60 frost attacks. So, here's the hit calculation:

0.83 (83% for +3 levels monster) + 0.06 (+6% spell hit) = 0.89
0.89*0.5 (50% damage from frost) = 0.445.

The game will roll a number between 0 and 1, and if it's less than 0.445, the frost bolt will hit for full damage. Otherwise, a resist message will appear.

2nd Example:

After the resist, Eyonix decides to fire a fireball at Yeti of Doom. Eyonix still has +6% spell hit. Fireball is not a binary spell. Here's the calculation:

0.83+0.06= 0.89

The game will roll a number between 0 and 1, and if its less than 0.89, the fireball will hit. Otherwise, a resist message will appear. After the fireball lands, the game will then apply spell resistance to determine a partial resist, if any. Assuming the yeti also has 50% fire resistance, on average, 50% of the damage will be resisted.

I hope that illuminates the impact of +spell hit gear on magical combat.

Oh wow man! I didn't realize it had that kind of impact! Where did you find this info?

Straight from the wow site, link's up top, scroll down a bit and thats where I got it from.