Assuming you're only off-tanking and never going to be a main tank, an arms/prot hybrid could work nicely for you. Choose the prot talents that maximize your threat and damage mitigation, and the arms talents that let you generate good burst DPS for extra threat generation.
My off-tank build was a hybrid based on this
prot package.
You can bolt that on to your favorite arms build and get a nice boost in damage mitigation and threat generation. I've seen a few people use fury/prot builds as well, and you can bolt the same 20 points of prot onto your favorite fury build if you prefer. This also lets you pick up the 41-point talent from your preferred tree, or pick up Concussion Blow from the prot tree - which works nicely in PVP and PVE alike :)
Cheers,
Jorge