What is the Defense Cap? It's actually a misnomer (a poorly-named phenomenon.) As previously stated in the blog, all raid bosses have a 5.6% chance to crit on level 80 tanks, and since defense skill reduces chance to be crit by .04% per point beyond 400, only 540 defense skill is required to make yourself 'uncrittable' by a raid boss (and all weaker NPCs, but not other players.) The bosses of Heroic dungeons have only a 5.4% chance to crit, and only require 535 defense skill.
So, why do you need to be uncrittable? Three reasons.
First, it's a really efficient way to reduce your incoming damage, when added to the parry, dodge and miss chance that Defense grants, reducing the opponents crit chance drastically cuts the total damage you take.
Second, it reduces the spikiness of your incoming damage. Healers like nice, steady amount of incoming damage, sudden unpredictable changes can catch them offguard and get everyone killed. Not to mention that, at the beginning of your tanking career when your health is comparatively low, and at the end when bosses hit 36k with their melee swing, a single crit from a boss can and will kill you from full health, and if there's even a chance of that, you gamble with the outcome of your raid.
Third, almost no one will let a PuG tank into their raid without checking for 540+ defense, and only some guilds will take applicants with less than that.
So, how to go about reaching this?
One way: Stack defense rating. Stack it like crazy. Don't even worry about your other tanking stats for now, just pile as much defense as you can, you can trade defense for other stats after you're defense capped. If you get something that's a great tanking item, but had less defense than your gear, put it in the bank until you can afford to wear it.
Here are some ways to kick your Defense up quickly, without farming (a lot of) nonheroics.
Crafted Gear - The Tempered Saronite set of items made by Blacksmiths is not only cheap, but packs a good amount of Defense on each piece. Unfortunately (at least, on my server) the other crafted defense items are not worth the money to buy them. I do not recommend trying to get jewelcrafted gear.
Questing - Many quest chains in Northrend give decent rares at the end, and although many don't include an item with DK tank stats on it, a few do. This is a link to the World of Warcraft Armory search page, I searched for DK tank items with 30 or more Defense Rating on them from quest rewards. Look up any of those items on Thottbot, Wowhead or another database site to find the quest chain it's from, do the chain, get the item. You'll probably score some nice XP and/or gold along the way and some rep with whichever faction.
I also hear that there is decent gear to be had from the Argent Tournament quests, but I have not checked it out myself.
Dungeon Quests - If you run a dungeon, be sure to do the quests associated with it. The Caverns of Time: the Culling of Stratholme, Ulduar: Halls of Stone, Utgarde Pinnacle and The Oculus all have their questgivers inside the dungeon itself, while Ulduar: Halls of Lightning's quests come from the surrounding zone, starting with They Took Our Men in K3. That one's a long chain, but will also unlock the Sons of Hodir rep and allow you to get those shoulder enchants. Gun'Drak's quests come from the village to the northwest of it, no chain required, and Violet Hold's quest comes from the Violet Citadel in Dalaran, starting with Discretion is Key. Don't worry about doing that chain in advance; the pre-quest is to walk over to Violet Hold itself. The other northrend dungeons are a bit low levelled to worry about for getting gear to tank Heroics or better, so don't bother with the rest.
Reputation - Questing and running level 80 dungeons (CoS, HoL, UP, Oculus, or any Heroic 5man) with the proper rep tabard will net you reputation gains, visit as many of the quartermasters as you can early on and plan out which ones you'll want gear from so you can coordinate which quest chains you do and rep tabard you wear while getting your other gear. Items available at Revered or lower can be considered as options prior to starting Heroics, and items at Exalted can be considered for before you begin raiding.
Runeforging - Rune of the Stoneskin Gargoyle is crazy good, giving 25 Defense skill. I didn't have access to this when I was gearing up, and we had to walk 20 miles in the snow to our dungeons, but it is possible to reach defense cap without it, it's just not a fun experience. Put this on every weapon you tank with until defense cap, after which you might consider other runes, but it's still a great option, even at 540 Defense, just for the 1% each of parry, dodge and miss chance.
PvP - Okay, so this won't get you to 540 defense, but it will get you to uncrittable. If you're really close, have a stockpile of Honor, and/or just like to PvP, some resilience can substitute for defense. 33 resilience = 10 defense skill for the purposes of reaching uncrittability, and can easily be gotten in the trinket slot, unlike defense. It will also protect you against DoTs, (but not in the next patch) although this is not that useful, it is another benefit. Normally, I would have said having resilience gear handy for a DoT-heavy fight might be a good idea, but since resilience is getting nerfed for PvE next patch, there's no longer any point.
Just don't stack a ton of resilience to get to uncrittable, resilience gear generally has primarily DPS stats on it, and is not as good as defense gear for tanking. If you're using more than 120 resilience to get your uncrittability, you're definitely overdoing it, and you should replace it all with PvE tank gear before leaving Naxx.
Once you have 535 Defense skill, you're good to go for Heroic 5mans. Once you pass 540 defense skill, you're ready to tank the first quarters of Naxxramas (VoA can be tanked as a rogue in DPS gear, so feel free to do that whenever.)