Yeah but they aren't squishy because of armor, not at all. If anything I would imagine the issue is the lack of blocking.
Armor increase will help though, wonder how much they will feel the need to buff it by.
I used this guide as I didn't want to rely too much on instance runs:
http://www.tankspot.com/forums/f97/40966-polar-s-easy-pre-naxx-tank-gear-list.html
On the issue of DK tanking, I find Frost Presence fine. I believe it's the fact that some of our essential tanking abilities are based on long CDs, and that with low dodge your Bone Shield will lose all its charges fast

I would rather have a longer duration of Icebound Fortitude or more charges on Bone Shield in exchange for less mitigation. But then again, it does help to sort out the good and alert healers from the slackers
When I used to tank on my Paladin, the hardest bit really was to spam Holy Shield every 8 secs. As long as I did that, I didn't have to worry about spike damage.
DK tanking has come a long way since I first tried it on the Beta, and I'm confident however Blizzard will sort it out. If not, I've got my Druid alt I can chill with
There are a few reasons I think.
1 of the main ones is DK's don't use a shield, and can't use a gun or a libram for example which Warriors and Paladins use.
This basically means they don't get the stats that the Shield has, on average a DK in Forst Presence has around the same armor as a Warrior for example carrying a shield with the same gear (give or take a bit - depending on the shield). However, they are missing the defence rating that the shield has to offer, which is a rather large problem, and the Libram also usually carries some kind of helpful bonus. (as well as a gem socket too usually, and the ton of HP and blocking damage the shield has to offer, obviously).
Warriors typically can get a massive HP boost from the gun + socketing the gun, so DK's lose a HP slot there, and while Paladins Librams are useful for tanking, so far the Sigils for a DK are rather lacking for that.
I am mostly going as a DPS DK, as my guild have 3 paladins which wish to tank (actually 2, plus 1 warrior) and that is with the 2 warrior tanks and 1 paladin we already currently have so tanks aren't an issue.
I have done a little tanking however for fun, or once when the server was randomly disconnecting people for a few days I had to take over as the tank in our 5 man group DC'ed in the middle of a boss. I did manage to tank the boss rather well in 5 man heroic, but to be fair I was getting hit with massive damage (I was DPS specced) but since he was down to around 40 percent and we had high DPS, I used all my cooldowns, and our healer wasa Disc Priest, so once he used Pain Supression on me too, the boss was down before I started to take to many crits.
Currently as I said again, DK's are not gods, and have large problems with tanking and a few with DPS, but then if you look at other classes they have their share of issues too.