Unholy DK tanks are rare mostly because folks don't know how to tank with Unholy. For now - which will change in Cata, of course - Unholy excels at AoE threat, far beyond the other two specs, IF:

1. You have D&D glyphed
and
2. You take the 2pT10 bonus

I've been tanking Unholy since 3.1 and have had no trouble generating or maintaining AoE and single target threat even in ICC. Granted, I've only got 50K HP, but combined with the Frost trinket (Disc-style mini-bubble), I've got 4 major CDs to use:

1. T10 Trinket bubble
2. Icebound Fortitude
3. Anti-magic shell - which is godly in any encounter with debuffs that can fall off, as it's a debuff cleanser if the debuff stops stacking if it stops ticking (like Hodir, Garfrost, etc.)
4. Anti-magic zone - your melee will adore you for this on some fights

My disc priest main healer and I enjoy playing a game on heroics to see if we can chain CDs together and go for 0 damage taken in the daily random emblem run if the DPS is solid. If we can get the boss to last for less than 30 seconds, I come out scratch-free.