Coming out of the Twitter developer chat tonight was this question and following answer, given by Ghostcrawler and J. Allen Brack:
While it's not a direct confirmation that warlocks will be getting a new pet in Cataclysm, I think it's safe to read into this and maybe get a bit excited about this one. I doubt they'd mention warlocks getting "new spells" in relation to getting a new demon if such an addition to the warlock class wasn't planned.Q. With the new skins for druid forms, what is the chance that warlocks might see race/faction specific demon skins or maybe a new one?
A. When you look around, there are a lot of say imp or voidwalkers we could use. It's something we talk about quite a bit. As for new demons, I hear warlocks are going to be getting some new spells for the next expansion...
So, new demons for warlocks? Probably. My guess is that it'll be a pet lava monster or something related to the elemental plane. Just the perfect kind of thing to pop up into the evil hands of warlocks from the sundered land in Cataclysm.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.