So my Warrior-esque character currently has a full suit of Daedric Armor (Legendary) that was enhanced via smithing and +smithing gear with a Blacksmith's Elixir (+50% to smithing enhancing).

My current Armor Rating with the full suit + shield is 1282 with no other enhancers besides the Heavy Armor perk tree bonuses. I have no + armor or heavy armor enchants.

At what point does armor rating become null? At what number does damage mitigation cease to increase from a higher armor rating? I've heard some people say it's in the 500's, I've heard it's in the 600's, I've heard the 700's, I've heard it never stops, so what's the truth?

567, after that any further tempering is useless

What source or proof can you back this up with?

It's 567.

Damage resistance is capped at 80%, and you reach that when at 667 armor rating. However, every piece of gear (head, chest, hands, feet) has a hidden +25 AR bonus, that's why the cap is actually 567.

damage reduction% = (displayed armor rating + hidden armor rating) * 0.12

Straight from UESP. I haven't checked it myself, but those guys know their numbers. And there have been a couple of topics here on the forums giving in-depth anolysis of skill caps.

http://www.uesp.net/wiki/Skyrim:Armor

Thanks for the link, Yonkers. It's actually funny because even without a shield my AR is over 1100 haha. Just too easy, IMO.

One last question:

Say I decide I want to go for looks and not wear a helmet. Say I just want to wear chest, gloves, and boots. No shield, no helmet. And my AR rating (as shown) is still 600. Am I still mitigating the full amount of damage because my shown rating is above 567?

One piece of armor is missing, so 667 - 75 = 592

As long as you stay above 592, you have maximum damage resistance.