I'm happy to share we actually do have plans for offering players a choice between male and female versions of all warlock demons. This is a task already in our system, though I don't have a time frame for you at the moment. Other art tasks could always take precedence, but we're committed to making this happen. :)
It looks like warlock pets will be getting all new art as well as choices, potentially through glyphs, as to whether their demons are male or female. After reading this post, I quickly began to think about what each pairing of pets could be. Voidwalker and the voidmistress? Imp and girl imp? Succubus and incubus? Mrs. Infernal! I am eagerly awaiting all of this new art.
We have no time frame for these new pet models, but I would not hold your breath for patch 4.1.
WoW Patch 4.1 is on the PTR, and WoW Insider has all the latest news for you -- from previews of the revamped Zul'Aman and Zul'Gurub to new valor point mechanics and new archaeology items.