OK, so I told a white lie in my post on racial abilities for Paladins, and it took me two weeks to get around to this post instead of one.
As before, I recommend to players who are just starting out to never choose a race simply based on its abilities, but rather based on how you enjoy looking at it.
Now, without further ado, let's get on to how your race selection effects the Warlock class.
Warlock - Horde
Undead - As with every class, the Undead racials Cannibalize and Will of the Forsaken are great for PvP, with the added benifit that Warlocks can turn the health from Cannibalize into Mana via Life Tap. Shadow Resistance, while not useless, isn't all that great, and Underwater Breathing is made moot by the Warlock's Unending Breath spell.
Verdict - Probably the best choice for a Warlock who plans on focusing on PvP
Blood Elf - While Magic Resistance gives little benefit to Warlocks, I'll actually act kindly towards Arcane Affinity. In my experience, a great number of Warlocks take up Tailoring and Enchanting as professions, and as a former enchanter, and extra 10 points is a welcome benefit. Mana Tap and Arcane Torrent are OK abilities, thought the Mana they steal and grant are negligible, and they should, for the most part, only be used in PvP.
Verdict - Another race that is stronger for PvP than PvE, though the Undead are still stronger in this regard.
Orc - Ah, I miss the day's of my Orc Warlock. Don't get me wrong, their racials aren't all that that and a can of fel energy. E.g. Axe Specialization; enough said. That aside, Blood Fury grants 143 spell damage, and Command works very well with Demonology Warlocks. As for PvP, Hardiness grants some protection from Rogues and Warriors, who are the bane of Warlocks everywhere.
Verdict - While OK for PvP, Orc are probably your best bet for PvE if you play Horde, and the best race anywhere if you plan on raiding as a Demonology Warlock (more on this in a later post).
Warlock - Alliance
Human - While Mace and Sword Specializations don't do anything for Warlocks, Diplomacy helps take the edge off of reputation grinds and Perception can help protect against Rogues and Cat form Druids in PvP.
Verdict - Probably the better choice for PvP.
Gnome - While Arcane Resistance, like all other racial resistance bonuses, isn't worth much, and Escape Artists isn't all that important for PvP since Warlocks don't need to be mobile to do damage, Expansive Mind is useful for PvE. And, for all you Engineering buffs out there, Engineering Specialization, at a whopping 15 points, will help take the edge off of leveling Engineering, letting you get your Destruction and Annihilator Holo-Gogs a little sooner.