Wrocas (European community manager) announced on the WoW forums moments ago that a guild chat feature will be added to the Mobile Armory that's available on both the iPhone and Android. Players have been asking for this type of feature ever since the armory was introduced. This feature piggybacks off the same service that the remote auction house uses, which requires an additional monthly fee on top of your normal World of Warcraft subscription. One fee to rule them all (and by "all," I mean both the mobile auction house and the guild chat feature).
This allows access to guild chat, officer chat (presuming you had access to start with) and whispers. It appears the whispers will only be allowed to and from guild members, at the moment. There is also no news about being able to chat with your Real ID friends. If you don't have a device that the Mobile Armory runs on, Blizzard will soon be adding this feature to the normal armory as well.
The full announcement is behind the break.