I'm just now getting into the theorycrafting and understanding how it works and what not. I've started building my own spreadsheets and tinkering with others to get the values I want. I realized earlier today that I didn't know how critical strike bonuses worked.

Anyone care to explain how untalented critical strike bonuses operate? I tried searching and to no luck.

I'd try wowwiki. Anyway:
melee: 200% damage on a critical (double). Rogues have a talent for 230% damage on special crits and warriors have a talent for 220%.
Casters: 150% (on non-DoT spells as those can't crit at all, if it's a mixed spell the dot portion of it can't crit but the rest can). That means crits are much weaker. However most casters that DPS generally have a talent to increase their crit back to 200% (or effective 210% for fire mages), with the exception of shadow priests and affliction warlocks. Healers also only crit for 150%.

Removed

Found pretty much what I was looking @ WoWwkiki. Alot of it was outdated but the theories apply.

What is the order in which damage added to critical strikes are calculated? As in how does PI effect the bonus. Is it additive?

Maybe you will find these two threads interesting, they're dealing with the Relentless Earthstorm Diamond and its 3% increased crit damage.

[Shaman] Relentless Earthstorm for Ele?
Relentless Earthstorm Diamond - melee only?

I read the wowwiki page about this: http://www.wowwiki.com/Formulas:Critical_hit_chance (look at the bottom of the page) and I'm a little confused about Spell Power, Ice Shards, and Ignite.

Here's the part that's confusing me:
The Spell Power description says that it "increases critical strike damage bonus of all spells by 50%." Does that mean that instead of 150% damage, spells do:

175% damage - 50% of the 50% damage bonus is added
or
200% damage - another 50% damage is added to the existing 150% damage due to the crit

The Ice Shards description says that it increases the critical strike damage bonus of your frost spells by 100%, so is that now 250% damage or 200% damage? I have a similar question about Ignite + Spell Power but ignite is easy to understand relatively.

What about with both Spell Power and Ice Shards? Do you get 300% damage from crits or some weird combination of 150% damage, 100% damage bonus, and 50% damage bonus? The Combat Mechanics post says nothing beyond 150% damage for spell crits before talents, and I can't figure out how Ice Shards + Spell Power is modeled in Vontre's spreadsheet. My mage is only 51 now so I can't try it myself yet.

I realize that Blizzard sometimes uses inconsistent wording on tooltips. Witness the wariety of ways +heal is indicated as "Equip: Increases healing done by magical spells and effects by up to x" versus "+50 healing." Is this just a case of inconsistent wording? I hope I'm not missing something really obvious.

The correct interpretation is:
The critical strike damage bonus for spells is 50% of the noncritial damage; 100% for melee. (Critical strike damage bonus = critical strike damage - normal hit damage.)
Talents that increase this increase the bonus, not the base damage, so Spell Power increases the 50% bonus by 50%, multiplicitavely, so the total bonus is 75%, meaning the total critical strike damage is 175%.