There are a couple of people (me included) on this forum that have level 80 DKs that we use as tanks for our team (in both cases I think at this point its a team of 4 shamans - me and Niley).

I've been running normal instances with my team for awhile but haven't done any Heroics yet. Will be trying some in the next day or two and will probably write up a DK tanking guide after that. I already have a good rotation for tanking I've been using for awhile so I will make sure that is still good for Heroics and then make a post. I actually have 2 rotations, one for bosses and one for trash. I use the one for bosses for non-instance content as well as it doesn't do any AoE threat really at all.