Death Knights are in the unique position of being open to every race (Warriors can choose any race but Blood Elf). This means you will have a lot of options for trying to pick the best racial bonuses for your new DK; complicating the matter is the fact that DKs can both tank and DPS. Milkgas, of Anvilmar (US), recently wrote in with the opinion that Dwarves would be optimal for a tanking DK, due to their frost resistance (speculating that frost will be prevalent in Wrath). But let's go over the options (listing only the racials that seem relevant to DKs):

• Horde:
  • Orc: Blood Fury would be handy for DPS, not so good for tanks; Hardiness, on the other hand, would be nice, if not essential, for a tank to have; and Axe Specialization is helpful, given that axes will be available for DKs to use as rune weapons (as well as swords and maces)
  • Tauren: War Stomp is great for PvP. Endurance is probably the best overall tanking racial out of all the races. And Nature Resistance will probably be handy somewhere, though it's not a huge deal.
  • Troll: Berserking is a useful, if occasional, boost.
  • Forsaken: I still contend that Will of the Forsaken is overpowered. May certainly come in handy while tanking, and of course in PvP. Shadow Resistance, like Nature Resistance, is a marginal benefit.
  • Blood Elf: It remains to be seen how the Mana Tap/Arcane Torrent system will work with DKs' Runic Power, but this has the potential to be very useful. Magic Resistance is, numerically, stronger than any other race's inherent resistance - you wind up with 25 points of resistance overall.