So I was browsing the forums over on elitist jerks and came up on this interesting tidbit, called the Crit cap. this is primamrily for dual wield classes like enhance shamans, rogues and fury warriors, but can also pertain to dead knights and paladins.

The theory and testing is as follows

Each potential hit is first checked against Miss, than Dodge, than Parry, than Block, than Glancing, than Crit, than hit ( this is added instead of subtracted and includes talents

Dodge and parry can be set to 0% by being expertise capped and attacking from behind. the miss change for dual wield is 27% (white hits, yellow are capped at 8%)
So to give an example, I have unbuffed a 42% crit, and 11.5% hit
this will give me 100%-27%-0% (exp capped)-0% (attacking from behind)-0%-24%-42+(11.5+6 from talents)=24.5% below the crit cap.

To shorten this up, if your expertise capped, and attack from behind,
your crit cap will be 100-27-24+your hit=your crit cap.

if you have any talents that increase your crit % for white hits, or any on proc trinkets that increase crit (agi does that), that will lower your crit cap accordingly. also ToW (totem of Wrath) puts a 3% crit debuff on the target which counts toward your crit cap.

The only way to increase your crit cap is to increase your hit.
The reason I am posting this is that I ran into the crit cap on several occasions, since I have a talent that can increase my crit by 9%, and with the raid buffs, I get to 62-63% crit. I am sure our rogues are close if not over the crit cap as well.

to see if your at or over the crit cap here is a macro to check it in game (includes totem of wrath)

EDIT,
After some smart folks did some testing on this subject, our hardcap for crit seems to be 80.8% (dual wielding). I modified the script so it takes the latest info into account, Since trying to explain this seems a bit to technical, I wont go into detail, but you can use this script to check your room before the critcap in raid.

/script DEFAULT_CHAT_FRAME:AddMessage("Room below crit cap: "..80.8-(27-(GetCombatRatingBonus(6)+6))-(GetCritChance()+3))
The +6 and +3 values are for hit talents and ToW

Gem for hit on a side note it is a soft cap and crit should still increase the damage from your yellow hits and spell damage. Now get to the hard cap

Your right, it is a softcap however gemming for hit beyond the 14% for spells is less desirable then gemming for AP or haste. Over 33% of my damage is done by white hits.

My specials like stormstrike contribute for about 7% of my damage, lava lash is 4%, but is subject to spell crit. Windfury is my second biggest damage source with about 14%.

Reason I posted this is that by the time you hit the softcap, more hit will help to raise that but it is not going to be as beneficial as attack power or haste.

Rogues are in the same boat as far as white hit goes, hoks damage is more then 40% white hits, but he has this ARP love thingy going.

Looking for the hardcap for melee (spells is another matter entirely) is 71.8% (white hits) yellow hits would be 95% or so