I'm running DK as my main, I will be tanking in raids once everyone's at 80. RIght now though, I'm running him with a group of 4 shamans, and it works really well. The class is definitely boxable as a tank, I highly recomend going deep Unholy if you are going to tank a box group with your DK, secondary talents being in Frost. I've been toying with doing a weird hybrid spec of Unholy/Frost, but just haven't had the time to play with it yet. Vyn's right though, all the tanks are capable now. So go with what feels right.