We tried frost DK - Ret pala - Holy Pala last week, and it felt like it could go somewhere.
Anyway, Frost DK gives some oppertunities when combined with a 2 paladin's, since you can slow, stun, interrupt and freeze your targets, and both ret and frost dk have insane burst damage.
Usually we'd go straight for healer, i use strangulate, if healer survives the strangulate it gets a stun and than its dead for sure :P
Also my general idea on that group was, nuke healer before they attack us, a paladin healer can take a hit or two, but in general people wont respond by killing YOUR healer if you nuke theirs :P