Hey there,
To begin I must say I HIGHLY suggest Unholy tanking for beginning heroic to entry level raiding (and perhaps further.) The spec does have a few weaknesses, but intelligent gameplay will overcome them.
This is my Unholy tanking spec.
http://www.wowhead.com/?talent=j0E0VZhZfghhxcoMIo0csut
It is designed to do massive threat on a single target, as well as holding staunch aggro on all adjacent mobs. In other words, this is a beast of a heroic running spec. I spent more than 4 hours doing the math on each talent point spent, doing a cost/benefit analysis of each decision possible.
And in the end the effort was worth it. I have gotten many, many, kudos on my tanking ability and average 9 heroics a day- mainly because I have drawn the attention of some really potent members of the server who enjoy running with me.
As mentioned above, this spec has one major weakness, and that is a lack of "Oh Shit" buttons. It only has two, Bone Shield and Icebound Fortitude. Both are on minute cooldowns, and provide -incredible- benefit. However, I have found that, with proper gearing, BS stays up much longer than I had anticipated (especially with the glyph), and using it as an "oh shit" button, instead of mindlessly spamming it each time it is up helps a lot on certain heavy hitting bosses.
In the end, I wouldn't suggest any Unholy spec for Main Tanking anything beyond entry level Naxx and heroics. Beyond that point, I highly suggest Frost, as it holds the banner in avoidance and mitigation, as well as more "Oh Shit" buttons than you can shake a stick at.
Hope this helps.
-Vamp