Warlocks are in. Source here.
The tiefling's inclusion in the core rules compelled us to imagine what the D&D world would be like with tieflings around in greater numbers. It also gave us added incentive to include the warlock class in the Player's Handbook, since tieflings and warlocks are an excellent match story-wise and flavor-wise. (Just so we're clear, tieflings can belong to any class in 4th Edition, not just warlocks. I'm playing a tiefling cleric in a current 4E playtest. His name is Zade Shadowhorn, and he worships Erathis, the goddess of civilization.)