I decided that such a narrow focus isn't the best thing for my blog, so this morning I respecced my offsec from frost DPS to frost tank.
My first impression in the new spec was wow this spec doesn't have many cooldowns. Looking at the usual spot on my actionbar I use for cooldowns, I had only three. Later I would find out that Unbreakable Armor is rediculous. It's even better than Icebound Fortitude, if that's even possible. However, it is on a longer, although still short, cooldown, and I was tanking a 5man Heroic, it may prove to be less useful in raids. That said, when I popped it on the slag in the second room of H HoL, it made me literally invincible. After that I used it on a lot of trash packs, it was still fully absorbing everything. Since it scales to your armor, Devotion Aura, Inspiration (useless in 3.2) and other effects will amplify the effect, and it will also preform better if your plate gear is of a higher item level.
So, after looking over the rest my spells, it came time to actually tank something. Wanting to use my new toys, I pulled with Howling Blast, and began to Obliterate and Frost Strike my primary target, blasting again whenever I got it. It didn't work. The Rogue in full tier 8 came and attacked the 'wrong' target, some others started AoEing even though it was only two mobs, and when I switched targets to grab the other, I lost the first one.
What I learned was that my prior advice is important even in a frost spec; "cast Death and Decay early and often" is as true in frost as it was in unholy, however it's not really necessary to spec into Morbidity. The normal 30 second CD was short enough to use it on every trash pack, and a fight only really needed a single D&D for me to retain threat on the off-mobs.
Another difference was that my single-target DPS was way up, but my threat was about the same. All in all, frost has lower overall threat, but higher single-target DPS. This is because Unholy's primary threat move, Death and Decay, has a good chunk of bonus threat on the side, but none of the frost spam attacks do.
My spec also had more Runic Power than I knew what to do with. I really like this, those three points in Scent of Blood and Chill of the Grave were totally worth it. Unlike in unholy, where Scent of Blood was just nice, in frost the proc effect would restore my Runic Power off nearly all my attacks, including my Runic Power dump. I almost couldn't empty my runic power bar if I wanted to. As a DPS, this would be wasteful, as a tank it's an excellent security net. Your cooldown needs Runic Power? Not a problem, bind Rune Strike to every key, and it'll still always be available.
A few pitfalls of the spec: you really can't do AoE threat anywhere near as well as an unholy tank can. You can't self-heal for beans. Death Strike almost isn't worth it in this spec, and you can't get Rune Tap properly. Also, since I just traded Herbalism for Jewelcrafting, I'm missing my Lifeblood button too, not that it was ever that useful (may be next patch). You can feel like you're spending a lot of time idle. If you D&D at the start of a fight, it may feel like you spend the next 4-5 global cooldowns not doing much, since your runes will be down. Don't worry about it though, D&D is pulling 3-4k threat per second on every target.
And some good things: Frigid Dreadplate and Improved Frost Presence puts the spec's passive mitigation higher than any other DK tank spec. As mentioned before, you'll do more damage than Unholy if you're on just the boss, making it die sooner. (Will compare to Blood eventually, gonna play with that spec next week maybe.) You can actually PvP. Acclimation is godly on Loken and Sapphirion. Of marginal utility on other fights, sadly.
Anyway, I'm going to continue tanking in Frost for a while, and see what I learn. If you're looking for a spec, don't copy the frost one I linked at the top of this article, it was a first draft, and after playing it, I can see that it needs some work.