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.

• Alliance:
  • Dwarf: Stoneform can be very useful to remove certain debuffs while tanking. If Frost ends up being a major damage type in Wrath, Frost Resistance might be the best inherent resistance to have, though this is still a small amount.
  • Gnome: Escape Artist is lovely for PvP. Also Arcane Resistance.
  • Human: Diplomacy is very pleasant to have in late-game PvE for faction grinding. Sword and Mace Specialization will both be usable to the Death Knight.
  • Night Elf: Quickness is good for a tank. Nature Reistance again, too.
  • Draenei: Gift of the Naaru is especially useful given that the DK is not a healing class (with the exception of various talented abilities like Blood Tap). As far as I know, it has not yet been revealed whether Draenei DKs will get Heroic Presence or Inspiring Presence, but either would be helpful. And Shadow Resistance, woo.

So which is best? My vote would be Orc for DPS, Tauren for tanking on the Horde side; and Human or Draenei for either role on the Alliance side. There are, of course, many reasons to choose a race other than racial bonuses, which after all tend to be fairly minor in the long run, but if you really can't decide, this might help give one race the edge. What race are you going to make your DK?