(For all Plate wearing tanks. Druids want as much Agi as they can get their grubby paws on.)
As of Patch 4.0 (the release of Cataclysm), Agility does not contribute to Armor. All it provides you is Dodge, and increased chance to Crit on physical attacks only - as a Death Knight, this is substantially less useful than actual crit rating, as it does not affect your diseases or Icy Touch. Each point of Agility provides roughly .002% dodge for a DK, before Diminishing Returns. It takes 430 points of Agility to gain 1% Dodge.
Strength, by contrast, provides large amounts of Attack Power (and it's attendant threat benefits), as well as increased chance to Parry. Specifically, each point of Strength is converted to .25 Parry rating, and it takes 176.71899 Parry rating to gain 1% chance to parry. Thus, it takes roughly 706 points of Parry to gain 1% Parry.
Here's the thing though: Neither contributes much of anything to survivability. More to the point, there's no such thing as good tanking gear with Agility. In short, there are no situations where you should have to choose between Str and Agi. If you're looking at an item with Agility for tanking, odds are you're looking at an item you shouldn't be. Either it's a gem or enchant (in which case you should be looking at neither Str, nor Agi, but rather Mastery, Parry, and Stamina), or it's a non-armor item (in which case, you shouldn't be looking at those rings/necks/trinkets with Agi/Crit/Haste, but rather at Str/Dodge/Parry/Mastery). And if you're looking enviously at Mail or Leather... well, you have other issues.
There are two Agility items worth looking at for level 85 tanks, and in both cases, it's because they're the only ilvl 372 (heroic raid) items in those slots that contribute Mastery and either Agi or Str before Lady Sinestra - and even then, the advantage of giving those items to a tank is... slim to none.